Wash the potatoes, cover and cook in water for about 15 minutes.
In the meantime peel and finely dice the onion. Wash, quarter and dice the tomatoes. Dice the bacon. Crumble toast finely. Drain mozzarella and cut into thin slices.
Wash the basil and shake dry. Pluck off the leaves and cut them into rough strips.
Drain, rinse and peel the potatoes. Fry bacon in a pot until crispy and take it out.
Heat 2 tablespoons of butter in the bacon fat. Sauté onion in it. Sprinkle with flour and sauté until light yellow while stirring. Stir in 3/8 l water, cream and stock until smooth and bring to the boil. Simmer sauce for about 5 minutes, stirring frequently.
Mix in bacon and basil. Season to taste with salt and pepper.
Grease 4 small pans or ramekins or 1 large mould. Pour in potatoes, tomatoes and sauce. Put cheese on top. Sprinkle with toast crumbs. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for about 20 minutes.