For sauce, in a medium saucepan, combine water, mushrooms, onion, red sweet pepper, catsup, sugar, butter, vinegar, Worcestershire sauce, chili powder, paprika, garlic, ginger, and black pepper. Bring to boiling; reduce heat. Boil gently about 30 minutes or until sauce thickens and is reduced to about 1-1/3 cups.
Meanwhile, thaw fish, if frozen. Rinse fish; pat dry. Brush fish lightly with oil; sprinkle with salt and pepper. Place on the rack of an uncovered grill directly over medium coals.* Grill for 4 to 6 minutes per 1/2-inch thickness or just until fish flakes easily with a fork, carefully turning once with a wide spatula.
Top fish with the hot sauce. Sprinkle with sesame seeds and serve with hot cooked rice or couscous. Makes 4 servings.
To test for medium heat, you should be able to hold your hand over the heat at food height for 4 seconds before having to pull away.