Combine flour, salt, black pepper and cayenne pepper. Dust fish pieces with flour mixture. Heat oil in a large skillet or wok over medium-high heat. Add fish pieces, a few at a time and brown evenly on all sides. Remove and keep warm.
Wipe out pan used for fish and heat oil over medium-high heat. Add onion and next 4 ingredients. Stir-fry 3 - 4 minutes. Add mushrooms and cook until soft. Add soy sauce and rice vinegar. Stir-fry until liquid is reduced to 2 – 3 tablespoons.
To serve, spread a thin layer of hoisin on a lettuce cup, place one piece of fish and a couple of tablespoons of the vegetable mixture onto the fish. Top with a dot or two of the chili sauce. Fold one edge over, roll and eat.