Cook pasta in salted water per package instructions until al dente. Drain well just before serving.
To make shrimp: Heat oil in a large 12 inch skillet. Add onion. Saute over medium-high heat for 1-2 minutes until softened. Add garlic cooking for 1 minute longer.
To the skillet add white wine or broth and butter. Simmer for 5-7 minutes over medium until slightly reduced.
Add shrimp, season with salt and pepper and red pepper flakes. Cook for 2 minutes over medium-high or until pink and fully cooked through. Do not overcook.
Drizzle shrimp with fresh lemon juice and toss.
Transfer drained pasta to plate and top with shrimp mixture garnishing with parsley and lemon slices.
Serve immediately garnished with parsley and lemon slices.