The spicy meat for this tostada recipe is prepared in the slow cooker. Layer the beef with your favorite fixings on warm tortilla shells to finish off this main dish.
Sprinkle steak with salt and pepper. Place 3 tablespoons of the lime juice, the garlic, jalapeño, onion, chili powder, cumin and cayenne in a slow cooker. Place beef on top and cook on HIGH for 6 hours or LOW for 8 hours.
Remove beef to a cutting board and when cool enough to handle, shred with fingers or using two forks. Place shredded beef in a large bowl.
Strain remaining liquid in slow cooker over a bowl or measuring cup, discarding solids. Add strained liquid and remaining 1 tablespoon lime juice to beef in bowl and stir to combine. Cover; keep warm.
Warm tostada shells in oven according to package directions. While tostadas are baking, heat refried beans in microwave for 1 1/2 minutes on HIGH, stirring halfway through until warm and spreadable.
Spread each tostada with 1 tablespoon refried beans. Place 1/3 cup beef over beans and divide lettuce equally among tostadas. Top each with 1 heaping tablespoon cheese and 1 tablespoon salsa.