In a large pot, bring water to a rolling boil and add a generous amount of salt. Add your spaghetti or fettuccine, cooking until al dente according to the package instructions, usually around 8-10 minutes. Drain the pasta and reserve a bit of the pasta water, then set it aside in a large mixing bowl.

While the pasta cooks, heat a large skillet over medium heat and melt half of the unsalted butter. Season the large shrimp with salt and black pepper, then add them to the skillet. Cook for about 2-3 minutes, until they turn pink and opaque. Carefully remove the shrimp from the skillet and set them aside on a plate.

In the same skillet, lower the heat slightly and add the remaining butter. Once melted, add the freshly minced garlic and red pepper flakes (if using), cooking for about 1-2 minutes until the garlic becomes fragrant but not browned.

Return the cooked shrimp to the skillet, along with the drained pasta. Squeeze fresh lemon juice over the top, and add half of the chopped fresh parsley. Toss everything together gently for about 1-2 minutes until well combined.

Taste the dish and adjust the seasoning with additional salt and black pepper if needed. Immediately plate your Garlic Butter Shrimp Pasta, garnishing with the remaining parsley and a sprinkle of freshly grated Parmesan cheese.