For the onion gratin, melt 2 tablespoons of butter in a saucepan. Stir in flour and sweat briefly. Add stock and milk while stirring. Bring to the boil, let it boil for about 5 minutes, season with salt and pepper. Grate cheese and melt in the sauce. Peel onions and cut into slices, except for 1. Mix sauce and onions and put into small casseroles. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 1 hour
Coarsely grind the pepper and sieve it in a fine sieve. Dab the meat dry, season with salt and turn in the ground pepper. Heat the oil in a pan. Fry the steaks for about 7 minutes while turning. Finely dice the remaining onion. Remove the steaks from the pan, wrap them in aluminium foil and let them rest
Sauté the diced onions in the frying fat until translucent. Deglaze with cognac. Add 2 tbsp. cold butter in small flakes to the pan while stirring. Arrange sauce, steaks and gratin and garnish with thyme
Preparation time approx. 1 hour 10 minutes