First of all prepare the ragu: wash, clean and finely chop the onion, celery, carrot, garlic and parsley. Sauté this mixture in a pan with the olive oil and the 3 basil leaves (coarsely ripped up by hand) over a low heat, stirring frequently to make sure it doesn’t stick to the bottom. As soon as it becomes a nice golden colour, tip in the two types of minced meat and mix thoroughly. When a sort of froth begins to form on the surface, season with salt and pepper, turn up the heat and pour in the wine. Once it has evaporated completely, add the tomato concentrate and stir it in well. Add the peeled tomatoes, squash them with a fork and stir again. Leave to simmer over a low heat for at least an hour, stirring from time to time. In the meanwhile prepare the pasta: pile the flour up on the work surface and make a “well” in the middle; break the eggs into it, add the oil and a pinch of salt. Start mixing slowly with a fork, moving the flour from around the edge into the centre, and then use your hands until the dough forms a smooth, compact ball. Roll it out into a thin sheet (roughly 2 mm thick) with a rolling pin, use a few handfuls of flour to make sure that it doesn't stick. Cut it into squares measuring roughly 10 cm and leave them to rest; it’s alright if they overlap, just sprinkle a little flour between them so they don’t stick together. Now make the béchamel sauce: melt the butter in a small, heavy-bottomed saucepan over a low heat. As soon as it has melted, add the flour and stir for 2-3 minutes with a wooden spoon, without letting the mixture brown. Pour a little of the milk (which you have brought to the boil) into the saucepan and mix it in well; add all the milk, a little at a time, stirring continuously. Cook for approximately 10 minutes, over a very low heat, taking care that the sauce doesn’t stick to the bottom of the pan. Turn the heat off and season with salt and nutmeg.
Cut the pecorino cheese into small cubes and coarsely chop the hazelnuts. Now you’re ready to prepare the cannelloni: distribute the ragu (which in the meanwhile will have finished cooking) over the squares of pasta, spreading it out with the back of a spoon; place 2-3 cubes of pecorino on each one and roll the pasta up to form the cannelloni. As you prepare them, arrange them side by side in an oven dish greased with olive oil. Distribute the béchamel and a few spoonfuls of ragu on top of the cannelloni, sprinkle the chopped hazelnuts over the surface and cook in a preheated oven, at 180 °C, for 20 minutes. Before serving, leave to cool down a little and garnish with the basil leaves.