1Heat oven to 400°F. Spray square baking dish, 8x8x2 inches, with cooking spray. If fish fillets are large, cut into 4 serving pieces.
2Place spinach on fish; sprinkle with garlic salt. Roll up each fillet, beginning at narrow end. Place rolls, with points underneath, in baking dish. Mix mayonnaise and mustard; spoon onto each roll. Sprinkle with crushed croutons.
3Bake uncovered 15 to 20 minutes or until fish flakes easily with fork. Serve with lemon wedges.