01 - Heat olive oil in a large skillet over medium heat. Add chopped onion and cook for 2 to 3 minutes until softened. Add garlic and cook for 30 seconds.
02 - Add ground beef to the skillet and cook, breaking it up with a spoon, until fully browned, about 5 to 7 minutes. Drain excess fat if necessary.
03 - Stir in cumin, chili powder, smoked paprika, oregano, salt, and black pepper. Add tomato paste and water, mix well, and simmer for 3 to 4 minutes until the mixture thickens.
04 - While the beef simmers, warm the refried beans in a saucepan over low heat, stirring occasionally.
05 - Spread a thin layer of warm refried beans over each tostada shell. Top with seasoned beef, shredded lettuce, cheese, and diced tomato.
06 - Garnish with sour cream, jalapeños, cilantro, and a squeeze of lime juice if desired. Serve immediately.