Place salmon in a greased 2-qt. microwave-safe dish; sprinkle with salt and pepper. Pour wine over top. Cover and microwave on high for 4-6 minutes or until fish flakes easily with a fork. Remove salmon and keep warm.
Add butter and onion to the pan juices; cover and microwave on high for 1 minute. Stir in the flour, mustard and tarragon until blended; gradually stir in milk. Cook, uncovered for 1-2 minutes or until thickened; stirring every 30 seconds. Serve with salmon.