Bring a large pot of salted water to a rolling boil. Add the spaghetti or fettuccine and cook until al dente, about 8-10 minutes. Drain and reserve a cup of pasta water.

In a large skillet, melt 2 tablespoons of butter over medium heat. Season the shrimp with salt and black pepper, add to the skillet, and cook for 2-3 minutes on each side until pink and opaque. Remove and set aside.

In the same skillet, melt the remaining 2 tablespoons of butter. Add minced garlic and red pepper flakes, and cook for 1-2 minutes until fragrant but not burnt.

Return the shrimp to the skillet along with the drained pasta. Pour in lemon juice and half of the parsley. Toss everything together, adding reserved pasta water as needed to achieve the desired consistency.

Taste and adjust seasoning with additional salt, black pepper, and red pepper flakes if desired.

Remove from heat, transfer to serving platter, and garnish with remaining parsley and Parmesan. Serve immediately.