Pat fillets dry with paper towels. Combine flour with next 4 ingredients. Dust fish with seasoned flour. Heat 2 tablespoons butter and olive oil in a large skillet over medium heat. Add fish and brown lightly on both sides until just cooked. Transfer to a dish or pan, cover and keep warm in a 200 degree oven.

Wipe pan out with a paper towel and return pan to a medium-hot stove. Add lemon juice, wine, sugar and capers. Bring to a boil and reduce liquid to 1 or 2 tablespoons. Remove pan from heat and whisk in butter and onions until butter is melted. Stir in tomato and spoon sauce over warm fish.