Thaw shrimp, if frozen. Peel and devein, if necessary. Rinse shrimp; pat dry. Place shrimp in a large self-sealing plastic bag. Add oil, garlic, tarragon, thyme, and Old Bay seasoning. Seal bag and turn to coat shrimp evenly. Marinate in refrigerator for 1 hour.
Prepare couscous according to package directions using the 1-1/4 cups broth in place of water. Stir tomatoes and onions into couscous. Set aside. In a bowl combine lemon juice and butter; set aside. In a large skillet cook and stir shrimp with marinade over medium heat for 3 to 5 minutes or until shrimp turn opaque. Pour half of the butter mixture over shrimp. To serve, divide spinach among individual bowls. Spoon couscous mixture on spinach. Arrange shrimp on couscous; spoon remaining butter mixture over all. Sprinkle with olives, parsley (if desired), and Parmesan. Makes 4 to 6 servings