I agree with ChefJune about adding extra seasoning to the Bechamel. However if you really want tomato sauce with the cannelloni.make the sauce a little thicker than usual, add lots of herbs and seasoning. you might put a layer of bechamel under the cannelloni and put the tomato sauce on top of the cannelloni. Then on top of the tomato sauce you could decoratively drizzle some more bechamel.The two sauces go well with each other.