Melt 2 tablespoons butter over medium heat in a small saucepan.
Add the onions and saute 2 to 3 minutes until tender.
Take pan off the heat; stir in the wine, grated lemon peel, and tarragon.
Set aside.
Preheat oven to 350 degrees.
Cut the red snapper fillets into serving pieces.
Arrange fish in a large buttered baking dish and sprinkle on salt and pepper to taste.
Tuck in nectarine slices.
Pour wine sauce over the fish and nectarines.
Dot with remaining 2 tablespoons butter.
Sprinkle with parsley.
Bake red snapper in preheated oven until fish flakes when pierced with a fork, about 20 to 30 minutes, depending on thickness of fillets.
When done, serve red snapper and nectarines with toast points or over a bed of boiled white rice, accompanied with pan juices.