Heat a skillet over medium heat.
Add ground beef and cook while breaking it apart.
Add diced onions and minced garlic to the pan.
Cook until beef is fully browned and onions are soft.
Drain any excess grease from the skillet.
Add chili powder, cumin, paprika, oregano, salt, and pepper.
Stir in tomato sauce or salsa.
Simmer for 5 minutes until the mixture thickens.
Warm the refried beans in a saucepan over low heat.
Lay tostada shells on a flat surface.
Spread 2 tablespoons of refried beans on each tostada shell.
Add 2–3 tablespoons of the beef mixture on top.
Sprinkle shredded lettuce over the beef layer.
Add shredded cheese on top of the lettuce.
Layer on tomatoes, onions, sour cream, or avocado as desired.
Serve immediately while the shells are crisp.