Peel and finely chop the onion and garlic. Peel, wash and finely dice the carrots. Heat 1 tablespoon of oil in a pan. Cook the onion, garlic and carrots at low heat for about 10 minutes until soft. Remove from the pan and allow to cool slightly.
In the meantime, pre-cook the cannelloni in boiling salted water for about 10 minutes. Coarsely grate the cheese. Wash, clean and slice the tomatoes. Pluck the oregano leaves from the stalks, wash, pat dry and chop finely. Knead minced meat with onion-carrot mixture and tomato paste. Season with salt and pepper. Heat 1 tablespoon of oil in a frying pan and fry the minced meat in it for about 5 minutes until crumbly. Remove and mix with half of the cheese and oregano. Remove the pasta carefully with a skimmer and rinse with cold water. Fill the pasta with the minced cheese mixture and place in a greased ovenproof dish.
Heat 1 tablespoon of oil in a frying pan and fry the minced meat in it for about 5 minutes until crumbly. Remove and mix with half of the cheese and oregano. Remove the pasta carefully with a skimmer and rinse with cold water. Fill the pasta with the minced cheese mixture and place in a greased ovenproof dish. Cover the cannelloni with tomato slices and sprinkle the remaining cheese on top. Bake the minced cannelloni in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 15 minutes. Arrange cannelloni on plates and garnish with oregano