Preheat your oven to 400°F (200°C). Spread corn tortillas on a baking sheet, brushing lightly with vegetable oil. Bake for 8-10 minutes until golden and crispy.

Heat 1 tablespoon of vegetable oil in a large skillet over medium heat. Add the diced onion and sauté until soft, about 3-4 minutes. Stir in minced garlic and taco seasoning, cooking for another minute.

Add the lean ground beef, breaking it apart with a spoon. Cook until browned, about 6-8 minutes. Stir in salsa and heat through for 2 minutes.

Heat refried beans in a microwave-safe bowl for 1-2 minutes. Stir every 30 seconds until hot.

Spread a layer of refried beans on each tostada shell. Top with seasoned beef, lettuce, tomatoes, avocado, and cheddar cheese. Add sour cream and cilantro before serving.