1. Preheat grill to high heat.
2. Arrange onion slices and bell peppers on a grill rack coated with cooking spray. Grill onions for 12 minutes, turning after 6 minutes. Grill bell peppers 12 minutes, turning occasionally. Remove onions and bell peppers from grill, and let stand for 5 minutes. Slice onion rings in half. Thinly slice bell peppers; discard stems and seeds. Combine onion, bell peppers, 1/4 teaspoon salt, and 1/8 teaspoon black pepper in a small bowl.
3. Sprinkle fish evenly with remaining 1/2 teaspoon salt and remaining 3/8 teaspoon black pepper. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add fish to pan, and cook for 3 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness.
4. Warm tortillas according to package directions. Divide fish, onion mixture, and jalapeño slices evenly among tortillas. Serve with lime wedges, if desired.