Slice onion half lengthwise into two quarters. Finely mince one of the quarters; reserve for salsa. Cut remaining onion into 1/4-inch-thick slices; set aside.
To prepare the salsa, combine minced onion, mango, 2 tablespoons cilantro, 1 tablespoon lime juice, salt, and cayenne pepper in a small bowl. Cover and refrigerate.
To prepare shrimp, heat oil in a large nonstick skillet over medium heat. Add sliced onion; cook 3 minutes or until tender, stirring occasionally. Add shrimp; sauté 3 minutes or until done. Remove from heat. Season with black pepper, 1 tablespoon lime juice, and 1 tablespoon cilantro.
To serve, cover tortilla with a damp paper towel; microwave on HIGH 20 seconds. Spread sour cream onto tortilla. Top with shrimp mixture and salsa.