Wash the potatoes. Cook in water for about 20 minutes. Clean, wash and halve the beans. Cover and cook in little boiling salted water for about 15 minutes. Drain. Quench potatoes, peel and let cool down
Peel and finely chop 1 onion. Heat 3 tablespoons of oil. Sauté onion in it. Deglaze with 1/4 l water, stir in vinegar and stock, bring to the boil. Cut potatoes into slices and pour the stock over them. Wash and roughly dice the tomatoes. Add the tomatoes to the potatoes and mix everything loosely. Season to taste with salt and pepper, cover and leave to stand for about 1 hour.
Wash the fillet, pat dry and cut into 16 medallions. Wrap each medallion with a slice of bacon. Clean, wash and cut the peppers into pieces. Peel 1 onion and cut into slices. Cut lemon into 4 pieces. Put 4 medallions, peppers and pieces of onion alternately on 4 skewers, finishing with 1 piece of lemon
Heat 1-2 tablespoons of oil in a frying pan. Fry the skewers on all sides at medium heat for about 10 minutes. Season with salt and pepper. Stir mustard and sour cream until smooth. Season to taste with salt and pepper. Season the potato salad again and arrange it with the skewers. Put the mustard cream as a blob on top and garnish if necessary