Preheat the oven to 190'C.
For the pommes boulangères: peel the potatoes and then thinly slice on a mandolin.
Peel and finely dice the onions.
Heat a large frying pan and then add the olive oil. Add the onions to the pan along with 3 or 4 sprigs of thyme. Sauté for 3 minutes, stirring every once in a while. Add the balsamic vinegar to the pan and continue cooking until all the liquid has evaporated. Mix the onions with the sliced potatoes.
Line the base of a large ovenproof dish with the potatoes and onions. Season with salt and pepper and then sprinkle over a few thyme leaves.
Bring the stock to the boil and then ladle it over the potatoes. Chop the butter into small pieces and sprinkle over the top. Transfer to the oven and bake uncovered for approximately one hour - check if the potatoes are cooked through using the tip of a sharp knife.
While the potatoes are cooking, you can prepare the rest of the dish.
Peel and slice the shallot. Chop the tarragon leaves and the flat parsley.
Place the white wine vinegar, white wine, whole white peppercorns, shallot and the tarragon stalks into a saucepan. Place on the heat to reduce by 3/4 and then strain the reduction.
Melt the butter in a separate pan to clarify.
Over a bain marie, whisk the egg yolks and white wine reduction until the mixture triples in volume and is light and fluffy (a sabayon).
Gradually add the melted butter to the sabayon and then whisk continuously to prevent the mixture from splitting.
Add the herbs, check the seasoning and set aside until ready to serve.
Cook the fillet steak and the green beans to coincide with the pommes boulangères being ready to serve.
For the steaks: heat a frying pan to hot and add a little olive oil. Season teh staeks with sea salt.
Colour both sides of the fillets and then remove from the pan. Cook for 5 to 8 minutes in the oven depending on the thickness of the steaks.
For the green beans: bring a pan of water to the boil and add a generous amount of salt.
Cook the beans for 3 minutes in the rapidly boiling water and then drain. Season with salt and pepper and serve with a knob of butter.
Serve the fillet steak alongside the pommes boulangères and the green beans. Finish the dish with the béarnaise sauce.
«Maris Piper, King Edward or Desirée potatoes work best for the pommes boulangères.»
Tender fillet steak served with green beans and pommes boulangères - thinly sliced potatoes layered with sautéed onions and thyme and then oven-baked for an hour and a half. This dish is finished with a home made béarnaise sauce.