Set each tortillas into muffin molds and form into cups (if needed, beforehand microwave tortillas a few seconds for a more malleable texture). Oven bake tortilla cups until it browns, for about 7 minutes. Remove from oven and cool down.
In a casserrole, pour milk. With a sieve, pour flour into fine rain or milk. Add while whisking until obtaining a smooth mixture. Bring to a boil until while constantly whisking until the texture thickers. Remove from heat. Add mustard, salt and pepper.
Boil salted water in a large casserole, cook asparagus for about 3 minutes. Cool down under cold water and drain. Set aside.
In a non-adhesive pan, heat oil at medium-high heat. Fry mushrooms until well colored, so for about 4 minutes. Add chicken and asparagus. Stir a few sconds and pour the preparation in bechamel sauce. Mix well and rectify seasoning if needed.