1. Stir together first 3 ingredients and 1/2 tsp. salt. Set aside.
2. Combine cornmeal, cumin, and remaining 1 tsp. salt. Rinse fish, and dredge in cornmeal mixture. Spray fish evenly with cooking spray.
3. Grill fish, covered with grill lid, over medium-high heat (350° to 400°) 3 to 4 minutes on each side or just until fish begins to flake with a fork.
4. Serve on buns with mayonnaise mixture, tomato slices, and shredded lettuce.