Serves 4 (serving size: 2 tostadas)
1. To prepare shrimp, combine shrimp, garlic powder, cumin, and 1/4 teaspoon salt in a bowl. Let stand 10 minutes.
2. To prepare black bean salsa, combine green onions and next 7 ingredients (through beans) in a medium bowl; toss to coat.
3. To prepare avocado topping, combine yogurt, juice, and avocado in a small bowl, mashing with a fork until smooth. Stir in tomato.
4. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Add shrimp to pan; grill 2 minutes on each side. Top each tostada with 1/4 cup bean mixture, 3 shrimp, and 1 tablespoon avocado mixture. Serve with lime wedges.