Peel and devein shrimp; set aside.
Place milk in a 1-cup glass measure; microwave at high 1 minute and set aside. Place margarine in a 1-quart glass measure; microwave at high 30 seconds or until melted. Add flour, stirring with a wire whisk; microwave at high 30 seconds. Gradually add milk, stirring until well blended. Microwave at high 2 minutes or until slightly thickened, stirring every 30 seconds. Stir in lemon juice and salt; set aside and keep warm.
Combine asparagus, peas, green onions, and pepper in a 2-quart casserole coated with cooking spray; microwave at high 1-1/2 minutes. Stir in shrimp and minced mint. Cover with casserole lid; microwave at high 3 to 4 minutes or until shrimp is done, stirring every 1-/12 minutes. Let stand, covered, 3 minutes; drain.
Combine shrimp mixture, pasta, and milk mixture in a large bowl; toss gently. Spoon onto serving plates; serve with lemon wedges. Garnish with mint springs, if desired.