Begin by thoroughly cleaning the leeks. Slice them into rounds and gently sauté in a drizzle of oil until they become tender.
Add the mushrooms to the pan, seasoning with salt and pepper. Sauté together until the mushrooms are tender, about 10-15 minutes.
Allow the mixture to cool slightly before transferring to a chopping board. Give it a rough chop.
Carefully spoon the filling into the cannelloni tubes and arrange them in a lightly greased casserole dish
For the béchamel, melt butter in a saucepan, then whisk in the flour. Cook this mixture gently until it's golden and nutty. Gradually pour in the milk, constantly stirring to create a silky, lump-free sauce. Season to taste.
Coat the cannelloni with the béchamel sauce. Bake in a preheated oven at 200°C (392°F) for about 45 minutes, or until golden and bubbly.
