Preheat: Heat oven to 190°C (375°F). Line a rimmed baking sheet with parchment paper and brush lightly with olive oil.
Cook the beef: In a skillet, heat olive oil over medium heat. Sauté onions until softened and lightly golden. Add ground beef, cook until browned, then drain excess fat. Return beef to skillet, stir in taco seasoning and ¾ cup water. Bring to a boil, reduce heat, and simmer for 5 minutes.
Assemble tacos: Place tortillas on the prepared baking sheet. Spoon beef mixture onto one side of each tortilla. Spread a spoonful of refried beans over the beef, sprinkle with cheese, and fold closed. Brush the tops lightly with olive oil.
Bake + broil: Bake for 15 minutes. Switch to broil for 3–5 minutes until the tops are golden brown.
Serve: Top with lettuce, tomato, sour cream, guacamole, salsa, or whatever you like. Serve immediately.