Preheat the oven to 450° F.
Melt butter in a small saucepan. Sauté onion until tender. Remove from heat. Add parsley, rosemary, thyme, salt and pepper.
Place fillets in a greased baking dish. Brush with herb sauce. Bake at 450° F until fish flakes easily when tested with a fork, about 8 to 12 minutes.
Transfer to serving dish. Garnish with lemon wedges.
Contributed by Joyce Taylor