In a saucepan, melt the butter over a medium heat and then add the flour. Whisk vigorously. Gradually add the milk and continue whisking until the sauce coats the back of a spoon. Season and add the grated truffle. Put to one side.
In a saucepan, brown the sliced mushrooms and then the sliced garlic in the truffle oil. Add the chopped parsley, followed by the salt, pepper and paprika.
Fill each cannelloni tube with the mushroom mixture.