Filling: In a large saute pan melt the butter and the oil and then add the onion and the spices. Cook until tender.
Add the tin of tomatoes and mascarpone then cook for 5 minutes. Add salt and pepper to taste. Take off the heat and add the breadcrumbs until you have a consistency similar to a stuffing. Place this aside to cool.
Sauce: In a saucepan over a moderate heat melt the butter then add the flour. Stir well until all the flour has been incorporated. Slowly, a little at a time, add the milk until you have a smooth white sauce. Whisk constantly. Add the tomatoes and cook for about 5 minutes.
Blitz the sauce with a hand blender or by transferring to a liquidiser. Taste and then add salt and pepper to taste.
To Assemble: Preheat the oven to 180 degrees C. Prepare an oven-proof dish by oiling it well.
Place a length of the spicy tomato-breadcrumb mixture along the centre of a tortilla and then roll it up. Place the filled tortilla in the prepared dish. Repeat with remaining tortillas. Pour over the tomato sauce trying to cover completely. Sprinkle over the poppy seeds.
Bake in the oven for about 25 to 30 minutes, or until bubbly.