Flan: Melt margarine and brown onion gently in it. Stir in flour then slowly add milk, stirring all the time. Once at a boil, add breadcrumbs, nuts and seasoning. Remove from heat, and press mixture into greased flan mould, so that it covers the base and sides. Bake in preheated oven for about 30 minutes.
Asparagus Filling: Melt butter, add mushrooms, tomatoes and asparagus. Season to taste and simmer gently, until warmed through and tomatoes are cooked. Drain off excess liquid and place in baked flan case (which should still be hot). Serve.