Bring large pot of salted water to a boil, cook spaghetti for 10 minutes or until almost done. Reserve ½ cup pasta water before draining and set aside.
Season the shrimp with salt and pepper. In a large saute pan, cook the shrimp in the oil and crushed red pepper over medium heat until pink and cooked through, about 3 minutes. Set aside and keep warm.
Reduce heat on saute pan to medium low. Add garlic and cook until just turning brown, about 5 minutes.
Add the cooked spaghetti, reserved pasta water, and parsley to the pan and stir to combine. Squeeze lemon and season with salt and pepper to taste.
Serve in large bowl with the shrimp. Garnish with Parmigiano Reggiano and extra crushed red pepper as desired. Serve immediately.