01 - Heat a large skillet over medium heat. Add ground beef and cook, breaking it up with a spoon, until browned and cooked through, about 5 to 7 minutes. Drain excess fat if necessary.
02 - Add chopped onion to the skillet and cook until softened, approximately 2 minutes. Stir in minced garlic and cook for an additional 30 seconds.
03 - Stir in ground cumin, chili powder, smoked paprika, dried oregano, salt, and black pepper. Add diced tomato if using. Cook for 2 minutes while stirring to combine flavors. Remove from heat and set aside.
04 - In a small saucepan, heat refried beans over low heat until warmed through, stirring occasionally to prevent sticking.
05 - Preheat oven to 350°F. Arrange tostada shells on a baking sheet and sprinkle evenly with shredded cheese. Bake for 3 to 4 minutes until cheese melts or warm shells without cheese if preferred.
06 - Spread a layer of warm refried beans onto each tostada shell. Top with seasoned ground beef and optionally sprinkle additional cheese on top.
07 - Garnish each tostada with shredded lettuce, diced tomatoes, sour cream, jalapeños, fresh cilantro, and a squeeze of lime juice. Serve immediately.