Melt butter in large skillet over moderate heat.
Stir in tarragon.
Add onions and mushrooms and cook gently, stirring constantly, 1 minute.
Add scallops and cook gently, stirring and turning, 4 minutes.
Sprinkle with wine and cook 1 minute longer or until just tender.
Serve sprinkled with chopped parsley and garnished with lemon wedges.