To make the cream: combine garlic, chipotle chilies, adobo sauce, juice from 1 lemon and cumin powder in a food processor. Blend to form a smooth paste. Add yogurt and pulse until thoroughly combined. Season with salt and pepper and set aside.
For the salsa, combine mango, scallions, cilantro, orange juice and salt and pepper in a medium bowl. Stir well. Just before serving, gently fold in avocado.
In a small mixing bowl, combine cumin, chili powder and salt and pepper. Brush fish with 1/2 tablespoon olive oil and coat with spice mixture. Let stand for 15 minutes.
Lightly brush grill pan or cast iron skillet with remaining olive oil and heat over high flame. Cook fish, turning once, 3 minutes per side for medium rare (or 4 1/2 minutes per side for medium). Remove from heat and slice into 1/2-inch thick pieces.
To assemble tostadas, arrange shredded lettuce and cubes of fish over tostada on a plate. Top with 3 generous tablespoons of salsa and spoon cream liberally over the salsa. Serve with lemon wedge.
Source: © 2008 National Mango Board