Using a bowl whisk together the wine, lemon juice, parsley, olive oil, pepper and salt to taste.
Add the onion slices.
In another bowl toss the red peppers, broccoli florets, and tomatoes; add about ¼ cup of the liquid from the onion mixture and toss the mixture well.
Spread the onion mixture in the bottom of a greased baking dish. Add the Flounder fillets on top. Cover it with parchment paper and bake it in the middle of a preheated 400F oven for 8 minutes.
Spread the vegetable mixture around the fish, cover with parchment paper and bake it for an additional 7-10 minutes, or until fish flakes.
Transfer the fish to a plate, arrange the vegetables around it, and spoon the juices over the top.