A sweet, tangy ragout enlivens this delicate, fast-cooking fish.
In a 10-inch nonstick skillet, heat 1/2 teaspoon oil over medium. Add onion and cook, stirring occasionally, until lightly browned, 1 to 2 minutes. Add tomatoes and cook, stirring occasionally, until slightly collapsed, 30 to 60 seconds.
Remove skillet from heat. Add vinegar; stir to combine. Season with salt and pepper. Remove to a bowl; cover and keep warm.
Wipe skillet clean with paper towels. Place 1 teaspoon oil in pan, and heat over medium-high. Season fish with salt and pepper; place in skillet. Cook, until lightly browned, about 2 minutes.
With a wide metal spatula, carefully turn fish over; cook until opaque throughout, about 1 minute. Transfer fish to a dinner plate. Spoon tomato-onion ragout over fish; serve with couscous or rice, if desired. Sprinkle parsley over top.
As you place the flounder into the skillet, tuck the tail of the fillet underneath to make it one thickness and ensure even cooking.