Scratch tomatoes crosswise, blanch with hot water. Peel the skin. Chop the tomatoes. Peel and chop onions and garlic. In 2 tablespoons of hot oil, fry diced onions and half of the garlic until transparent.
Add the diced tomatoes, tomato paste, salt, pepper, 1 pinch of sugar and rosemary needles. Simmer for about 10 minutes. Clean, wash and chop the peppers. Steam in remaining oil with remaining garlic for about 8 minutes.
Wash and chop the basil. Drain tuna and pluck into pieces. Add both. Bring milk, sour cream, chicken stock, salt, pepper and nutmeg to the boil. Season to taste and thicken with sauce thickener.
First pour 1/3 of the tomato sauce into a lightly greased casserole dish. Then cover with lasagne plates. Place 1/3 of the pepper and tuna mixture on top. Place lasagne platters on top again. Layer everything one after the other and finally add béchamel sauce to the casserole.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 30-35 minutes. Decorate as you like with some pieces of pepper, basil and rosemary needles.