Prepare grill or broiler. Combine chili and onion powders, and salt and pepper to taste in a bowl. Sprinkle on both sides of fish. Grill or broil 6 inches from heat, turning once, 8-10 minutes or until fish flakes easily. Combine sour cream and scallions in a bowl. Place fish on bottom muffin halves. Top with cucumber slices, sour cream mixture and remaining muffin halves.