Peel and finely dice the onion and garlic. Heat the oil in a pan and fry the onion until transparent. Add garlic and steam briefly. Chop the canned tomatoes with a long knife in the tin and put them into the pan. Season with salt, pepper and sugar. Let it boil down at medium heat for about 10 minutes.
In the meantime, pluck the parsley leaves from the stalks, wash, pat dry and chop finely. Wash the chives and cut into fine rolls. Mix ricotta with the chopped herbs. Season with salt and pepper. Pluck the marjoram leaves from the stalks, wash, dab dry and chop finely. Wash and halve the cherry tomatoes. Remove tomato sauce from the heat, stir in chopped marjoram and cherry tomatoes. Fill the ricotta mixture in a piping bag without spout and squirt into the cannelloni. Put half of the tomato sauce in an ovenproof dish, place the cannelloni on top and pour the rest of the tomato sauce over them. Drain the mozzarella and cut into slices. Place the mozzarella slices on the cannelloni and cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 45 minutes. Cover after 30 minutes if necessary.
Remove tomato sauce from the heat, stir in chopped marjoram and cherry tomatoes. Fill the ricotta mixture in a piping bag without spout and squirt into the cannelloni. Put half of the tomato sauce in an ovenproof dish, place the cannelloni on top and pour the rest of the tomato sauce over them. Drain the mozzarella and cut into slices. Place the mozzarella slices on the cannelloni and cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 45 minutes. Cover after 30 minutes if necessary. Take out the cannelloni. Serve with marjoram