In a frying pan over a medium heat, warm a little oil then add the onion and garlic then sweat off, for about 4 to 6 minutes. Add the minced beef and brown, breaking apart with a wooden spoon to get rid of any lumps. Mix in the chilli powder, chopped tomatoes and kidney beans then season and cook until the beef is no longer pink.
Warm the tortillas in a dry frying pan over a medium heat and keep warm.
In a saucepan over a medium heat, warm the bechamel sauce and add a little mustard. Add a little at a time until desired flavour is achieved.
Fill the tortillas with the beef mixture, roll up and place in an ovenproof baking dish. Top with the bechamel sauce and place under a hot grill until bubbling and golden brown, about 6 to 8 minutes. Remove from the heat and serve hot.