Step 1: Prepare the Shredded Beef : Heat olive oil in a large skillet over medium-high heat. Season beef with salt and pepper, then sear each side until browned (about 2–3 minutes per side). Transfer to a slow cooker or pot. Add diced onion, garlic, cumin, chili powder, paprika, oregano, tomato paste, and beef broth. Cover and cook on low for 6–8 hours (or high for 3–4 hours) until the beef is tender. Shred the beef using two forks and stir in lime juice. Keep warm.
Step 2: Make the Salsa Verde : Boil tomatillos, jalapeño, onion, and garlic for 5 minutes until softened. Transfer to a blender, add cilantro, lime juice, and salt. Blend until smooth. Adjust seasoning to taste.
Step 3: Assemble the Tostadas : Spread a layer of warm refried beans on each tostada shell. Add a generous portion of shredded beef. Drizzle salsa verde over the top. Add shredded lettuce, cheese, and a dollop of sour cream. Garnish with fresh cilantro and a squeeze of lime juice.