01 - Heat olive oil in a large skillet over medium heat. Add chopped onions and cook for 2 to 3 minutes until translucent. Stir in minced garlic and cook for an additional 30 seconds until fragrant.
02 - Add ground beef to the skillet, breaking it apart with a spoon. Cook until browned, about 6 to 8 minutes. Drain excess fat if necessary.
03 - Mix chili powder, ground cumin, smoked paprika, dried oregano, salt, and black pepper into the beef. Cook for 1 minute to toast the spices evenly.
04 - Pour in tomato sauce and reduce heat. Simmer for 3 to 4 minutes, stirring occasionally, until thickened and flavorful. Remove from heat.
05 - In a separate saucepan, heat the refried beans over low heat. Stir in water gradually to achieve a spreadable texture. Keep warm until assembly.
06 - Spread a layer of warm refried beans on each tostada shell. Top with seasoned beef, shredded lettuce, diced tomatoes, sliced red onions, cheese, avocado slices, and chopped cilantro.
07 - Drizzle each tostada with sour cream and serve alongside lime wedges.