Heat olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3–4 minutes until softened and translucent.
Add the garlic and cook for another 30 seconds, stirring constantly so it doesn't burn.
Add the ground beef to the skillet, breaking it apart with a wooden spoon. Cook until no pink remains, about 6–8 minutes. Drain excess fat if needed.
Sprinkle in the chili powder, cumin, smoked paprika, oregano, cayenne, salt, and black pepper. Stir everything together until the beef is evenly coated in the spices.
Stir in the tomato paste and beef broth. Let it simmer for 3–4 minutes until the mixture thickens slightly. Remove from heat.
Spread a thin layer of warmed refried beans onto each tostada shell.
Top with a generous scoop of the seasoned beef, then pile on the cheese, lettuce, tomato, sour cream, and avocado.
Finish with fresh cilantro and a big squeeze of lime. Serve immediately while the shells are still crispy.