Heat oil in a medium skillet, and saut? onion and garlic in until soft. Stir in beans, and mash. Gradually stir in water and heat until warm. Remove from heat, and stir in the chilli powder, cumin, mustard, cayenne pepper and soy sauce.
Divide bean mixture and mashed sweet potatoes evenly between the warm flour tortillas. Top with cheese. Fold up tortillas burrito style.
Bake for 12 minutes in the preheated oven, and serve.