In a large bowl, combine onions, green and red sweet pepper, mayonnaise, mustard, garlic, lemon juice, drained capers, seasoning, and cayenne pepper. Add fish and bread crumbs. Mix well. Shape trout mixture into twelve 2-1/2- to 3-inch diameter patties.
In a large skillet, heat olive oil over medium heat. Cook trout cakes, half at a time, in hot oil for 4 to 5 minutes or until lightly browned and heated through, turning once halfway through cooking. Transfer trout cakes to an ovenproof serving platter or cookie sheet. Keep warm in a 300 degree F oven while cooking remaining patties. (If necessary, add more oil during cooking.)
If you like, serve trout cakes with lemon wedges. Makes 12 trout cakes.