Peel and finely dice the onions and garlic. Cut chillies lengthwise, remove seeds, wash and chop finely.
Heat the oil in a large pot. Fry the onions, garlic and chilli in it. Stir in the tomato paste, sweat briefly. Add tomatoes and juice, 1/8 l water and stock, chop tomatoes a little bit.
Bring everything to the boil, cover and simmer for about 10 minutes. Season to taste with salt, pepper and sugar.
In the meantime, rinse and drain the beans. Drain the tuna and tear with a fork. Grate mountain cheese. Cut mozzarella into thin slices. Stir the beans and tuna into the tomato sauce.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2). Spread some sauce thinly on the bottom of a square casserole dish. Alternately layer lasagne dishes, sauce and mountain cheese, up to 2 tablespoons.
Finish with lasagne and a little sauce. Cover with mozzarella and sprinkle with 2 tablespoons of mountain cheese. Bake in a hot oven for about 45 minutes.