Cook spaghetti to desired doneness as directed on package. Drain; cover to keep warm.
Meanwhile, melt margarine in large skillet. Stir in flour, thyme, salt and pepper; cook until mixture is smooth and bubbly. Gradually stir in milk; cook until mixture boils and thickens, stirring constantly.
Add fish and vegetables. Bring to a boil. Reduce heat; cover and simmer 5 to 10 minutes or until fish flakes easily with fork. Serve over spaghetti. Sprinkle with parsley.