Peel and chop the onions and garlic. Clean, wash and cut the chilli pepper into rings. Heat the oil in a pan. Sauté onions, garlic, basil and chilli in it. Add tomato puree and sauté briefly.
Deglaze with tomatoes and stock. Bring to the boil, season with salt, pepper and sugar. Simmer for about 6 minutes at medium heat. In the meantime, pour the beans into a sieve, rinse with cold water and drain.
Pour the tuna into a sieve and drain. Grate mountain cheese. Cut mozzarella into thin slices. Grease an ovenproof dish (19 cm wide, 30 cm long and 6.5 cm deep). Mix the beans with the tomato sauce.
First spread some tomato sauce thinly in the mould, then alternately layer lasagne plates, sauce, tuna and mountain cheese, up to 50 g for sprinkling. Finish with a thin layer of sauce. Cover with mozzarella and sprinkle with 50 g mountain cheese.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 45 minutes. Remove from the oven and garnish with chilli pepper and basil.