Chop onion
In large skillet, heat vegetable oil on medium
Add ground beef to preheated skillet
Cook, stirring frequently, 5 minutes
Add chopped onions
Keep cooking until their is no pink in the meat
Add refried beans, tomatoes, and water
Sprinkle taco seasoning over mixture
Stir until all ingredients are completely mixed
Cook an additional 3 minutes to ensure all ingredients are heated
Shred lettuce
Combine meat mixture, lettuce, and cheese on tortilla
Serve