1/2 cup oil-cured black olives (kalmata), pitted, halved
Position rack in center of oven and preheat to 400°F. Boil wine in medium saucepan until reduced to 1 cup, about 5 minutes. Remove from heat and reserve.
Generously brush 18x12x1-inch baking sheet or roasting pan with olive oil. Top with white onion (Vidalia) slices in single layer. Sprinkle with salt and freshly ground black pepper. Drizzle 4 tablespoons oil over onion
Rinse fish inside and out; pat dry with paper towels. Sprinkle fish inside and out with salt and freshly ground black pepper. Lightly dust outside of fish with flour. Pour enough olive oil into extra-large skillet to reach depth of 1/4 inch; heat over medium-high heat until oil is very hot. Working with 1 fish at a time, add fish to skillet and pan-fry until golden crust forms on skin, about 3 minutes per side. Carefully place fish atop onion on baking sheet or roasting pan Gently stuff cavity of each fish with 2 crushed garlic cloves, then 1/4 cup chopped parsley. Pour reserved wine over onions on baking sheet or roasting pan.
Roast fish uncovered 35 to 40 minutes. Scatter tomato and olives around fish; bake until fish is just cooked through, about 15 minutes longer. Transfer fish to large platter; cover with foil to keep warm.
Increase oven temperature to 475°F. Continue to bake uncovered until tender and tomatoes are very soft and beginning to color in spots, about 15 minutes longer.
Arrange tomato mixture around fish on platter. Serve when ready!