For the gratins, peel onions, except for 1, and cut into slices. Grate the cheese. Heat 2 tablespoons of butter in a saucepan. Sauté the flour lightly in it. Stir in 200 ml water, milk and stock. Bring to the boil and simmer for about 5 minutes.
Melt the cheese in the sauce. Season with salt and pepper and mix with the onion slices. Pour into four small or one large casserole dish. In a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see
manufacturer) bake for about 1 hour.
For the steaks, crush peppercorns in a mortar. Put the pepper through a fine sieve so that the pepper dust does not burn when frying.
Pat the steaks dry. Season with salt and crushed pepper, press on a little. Heat oil in a pan. Fry the steaks in it on medium heat for 3-4 minutes on each side. Wrap in aluminium foil and let it rest.
Peel the remaining onion, dice finely and fry in hot frying fat until translucent. Deglaze with cognac. Add 2-3 tbsp. ice-cold butter in small pieces and fold in until the sauce thickens slightly (do not boil any more!).
Season the sauce with salt and pepper. Arrange everything and sprinkle the steaks with salt flakes.