Preheat oven to 350. Brown ground beef and onion in a large non-stick skillet, breaking beef into small crumbles. Add tomato sauce, mustard, garlic powder, and onion powder. Stir to combine.
Take one flatbread and top with spinach, ground beef, and cheese. Place on baking sheet and repeat with remaining flatbreads.
Bake flatbreads until cheese melts, about 5 minutes.
Recipe by Mom to Mom Nutrition at https://momtomomnutrition.com/food-and-recipes/cheeseburger-melt/