Fillet Steak with a Herby Lemon Crust and Béarnaise Sauce

It's not easy pleasing a family with divergent tastes, so I was delighted when my teenage sons, after their first bite of this meal, fell off their chairs in a pretend faint. I adore fillet steak, and béarnaise is probably my favourite sauce in the world, but as the former is ludicrously expensive, and the latter indecently rich and fatty, these are not things we eat every day.

But my husband was home for the weekend, after an absence of a month, and I wanted to make something splendid. This combination of rare, meltingly tender fillet steaks topped with crunchy, herby, lemony crumbs and served with béarnaise sauce - a silken emulsion of butter and egg yolks flavoured with tarragon and reduced vinegar - is quite scrumptious. And perfect for a dinner party.

This sauce often strikes terror into the hearts of cooks and chefs, because it is temperamental and can curdle in the blinking of an eye. But, although time-consuming to make, it is worth the effort, and quite easy to master if you are patient and methodical in making it.

The trick with béarnaise is to take your time over it (this sauce definitely requires at least 15-20 minutes of loving attention), to whisk it diligently, and to make sure that the mixture heats very, very slowly. You can do this by putting a heat diffuser over a hot plate, and placing a heavy-bottomed saucepan on top of that, or you can make the sauce in a double boiler. By far the best method, in my view, is to place a big roasting pan of warm water on top of your hob, with half the pan over the hot plate (or gas fire) and the other half resting on the adjacent, cold plate. Put a glass or metal bowl into the cool end of the roasting pan and gradually move it towards the hotter side. By the time you've beaten in all the butter, the bowl should be in the hottest part of the water. Also, it helps to keep dipping your finger in the sauce as it heats. When the béarnaise is ready, it will feel hot to your finger, but not unbearably hot (which will scramble the eggs). If it feels lukewarm, it's not cooked.

If your béarnaise looks like it's starting to separate, you can save it by quickly beating in some cold water, a tablespoon at a time. If the eggs have curdled, though, it's too late to save, and you will need to start all over again. Traditionally, béarnaise sauce is made with clarified butter, but I find it works just as well with slightly softened unsalted butter.

If you can't find fresh tarragon (and I often can't in Johannesburg), use a good brand of dried tarragon, or, as I did, crumbled frozen tarragon (read my notes here about freezing dill; the same applies to tarragon).

Ideally, a béarnaise sauce should be made just before it is served but this isn't always practical. If you need to put the sauce on hold for a while, place a piece of clingfilm directly on the surface of the sauce (to prevent a skin forming) and leave the bowl resting in hot water. Even better, tip the sauce into a wide-mouthed thermos flask and screw on the lid tightly. First prize, though, goes to the wonderful Wonderbag: a polycotton drawstring bag filled with polystyrene beads that holds food at a constant temperature for hours. My wonderbag held a batch of this sauce for over an hour with absolutely no loss of texture or flavour. Click here to read my post about this brilliant South African innovation.

Fillet Steak with a Herby Lemon Crust and Béarnaise Sauce

1 whole fillet steak, at room temperature, cut into 3-cm thick slices
salt and milled black pepper
oil and butter for frying

For the crust:
3 slices day-old white bread
2 T (30 ml) fresh oregano or thyme, or a combination
2 T (30 ml fresh parsley leaves
1 T (15 ml) fresh sage leaves
1 t (5 ml) fresh rosemary needles
the finely grated zest of a lemon
3 T (45 ml) slightly softened butter
salt and freshly milled black pepper

For the béarnaise sauce:
4 T (60 ml) white-wine vinegar (or white balsamic vinegar)
1/4 of an onion (or 1 shallot), peeled and finely chopped
a small bayleaf
a sprig of tarragon (dried will do; see notes above)
a sprig of fresh thyme
8 peppercorns, lightly crushed
3 small egg yolks, or two jumbo ones
1 T (15 ml) cold water
180 g unsalted butter, softened
lemon juice to taste
salt and milled black pepper
1 t (5 ml) chopped fresh tarragon
1 t (5 ml) chopped fresh chervil (or parsley)

First make the crust. Put the bread, the herbs and the lemon zest into a food processor fitted with a metal blade and process until you have a fairly fine crumb. Add the butter, whizz a few times to combine, and season with salt and pepper.

To make the sauce, put the vinegar, onion, bayleaf, thyme, tarragon and peppercorns into a saucepan. Heat the mixture to a gentle simmer, and cook until the vinegar has reduced by two-thirds (you should have about 4 t - 20 ml - remaining). Remove from the heat, stir well, and set aside to cool completely. Place a roasting pan full of warm water on top of the hob, half off a hot plate and half on (see notes above). Allow to heat slightly, and place a glass bowl in the cooler end of the pan. Put the egg yolks, water, a pinch of salt and 1 T (15 ml) cold water in the bowl. Strain the cooled vinegar into the pan. Beat, using a balloon whisk, for 10 minutes, or until slightly thickened, slowly moving the glass bowl towards hotter side of the roasting dish so that the mixture gradually warms up. Now add the soft butter, a knob at a time, whisking continuously as the sauce thickens and gradually heats. Don't allow the water around the bowl to boil. When all the butter has been incorporated, and the mixture is thick, hot and smooth, remove from the heat and add - if you're serving the sauce immediately - a squeeze of lemon juice, to taste, and the chervil and tarragon. If you're going to keep the sauce warm for a while, add the lemon juice and chopped chervil and parsley just before you serve it. Check seasoning.

Heat a large non-stick frying pan or skillet, add a little oil and, when blazing hot, add the fillet steaks and a knob of butter. Cook, in batches, for two minutes or so, on either side (according to how pink you like your steak), until nicely browned on the outside. In the meantime, heat the grill on your oven.

Remove the steaks from the pan, place on a baking tray and cover each one with a little of the prepared herb crust. Pat down gently so that the crust sticks, but not so hard that it compresses into a carapace. Put the steaks under the grill for a minute or so - not too close - and watch them like a hawk. When the crumb topping is golden and crispy, remove from the oven and set the steaks aside to rest for exactly five minutes.

Serve with the béarnaise sauce, stir-fried spinach and perhaps some creamy scalloped potatoes.

Serves 6-8, depending on the size of your fillet.

Note: You can add any combination of fresh herbs to the topping, but I think sage is just essential. For a change, try adding a little Dijon mustard and fresh chopped garlic.

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