First, peel the onion and chop as finely as possible. Peel the garlic clove and press it through a garlic press.
Blanch tomatoes in hot water, rinse in cold water and peel, then finely dice.
Wash basil, pluck and chop finely.
For the Bolognese sauce, heat oil in a pan. Sauté onion and garlic until translucent, add tomato paste and minced meat and fry until crumbly. Add polpa and diced tomatoes and let the sauce simmer for a few minutes.
Meanwhile, for the bechamel sauce, melt butter in a saucepan. Sprinkle in flour and toast briefly over medium heat (it should not brown).
Pour half of the milk into the flour-butter mixture and whisk thoroughly. Allow to thicken briefly, then stir in remaining milk with Parmesan, allow sauce to thicken again and finally season with salt and a little nutmeg.
Stir the basil into the Bolognese sauce and season to taste with salt and pepper. Let it cool down a bit. Preheat the oven to 180 °C. Grease a baking dish.
Fill the cannelloni with the sauce using a pastry bag (or a teaspoon). Then place them close together in the prepared baking dish, pour the béchamel sauce over them and sprinkle with Parmesan cheese.
Bake in the preheated oven for about 45 minutes.