Heat oil in a medium skillet and saute onion and garlic until soft. Mash beans into the onion mixture. Gradually stir in water; heat until warm, 2 to 3 minutes. Remove from heat and stir in the soy sauce, chili powder, mustard, cumin, and cayenne pepper.
Divide bean mixture and mashed sweet potatoes evenly between the tortillas; top with cheese. Fold tortillas burrito-style around the fillings and place on a baking sheet.
Bake in the preheated oven until warmed through, about 12 minutes.