Stir 80 g butter for about 5 minutes until creamy. Wash the lemon, grate dry and grate half of the peel. Add lemon zest, cognac, pepper and 2 pinches of salt to the butter. Put the butter on a piece of cling film and form into a roll (approx. 4 cm Ø). Chill butter for about 30 minutes. Put potato balls on a baking tray.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-24 minutes. Clean, wash and cook the beans in boiling salted water for about 7 minutes. Pour off the water and briefly pour cold water over the beans. Tie the fillet of beef with kitchen string and season with salt and pepper. Heat oil in a pan. Sauté the steaks on both sides. Continue cooking over medium heat for about 6 minutes, turning the steaks once. Peel onion and cut into fine cubes. Heat 1 teaspoon butter and sauté the onion for about 2 minutes. Wash the thyme, dab dry and remove the leaves. Add beans and thyme to the onions, season with salt and pepper and sweat for 1-2 minutes.
Sauté the steaks on both sides. Continue cooking over medium heat for about 6 minutes, turning the steaks once. Peel onion and cut into fine cubes. Heat 1 teaspoon butter and sauté the onion for about 2 minutes. Wash the thyme, dab dry and remove the leaves. Add beans and thyme to the onions, season with salt and pepper and sweat for 1-2 minutes. Cut butter into slices and place on the steaks. Serve the steaks with the beans and potato balls