Heat the olive oil in a pan with a piece of butter and the garlic. Then add the mushrooms and cook on low heat, covered, for about 15 minutes. At the end of the cooking time, add salt, pepper and finely chopped parsley.
In another pan prepare the Béchamel sauce: melt 100 g of butter, then add flour and wisk together making sure no clumps arise. Gradually add the hot milk until the desired consistency is reached. Then remove from heat, add the grated hard cheese and mix well.
Clean the leeks, cut them into slices, heat the olive oil in a pan and fry them a bit. Simmer for a few minutes and then pour over half a glass of hot milk. Once the leeks have softened, turn off the heat and let cool.
Set aside 1/4 of the Béchamel sauce and mix the rest in a bowl with the porcini mushrooms and leeks. Then put the filling in a piping bag and fill the cannelloni with it.
Cover the bottom of a casserole dish with the Béchamel sauce set aside, place the cannelloni on top and cover with more Béchamel sauce. Then sprinkle grated hard cheese.
Bake the cannelloni at 180°C for about 30 minutes, let cool a bit and serve.