(If using pasta): Bring a large pot of generously salted water to a boil. Cook linguine according to package directions until al dente. Important: Reserve ½ cup of starchy pasta water before draining. Drain the pasta and set aside.
Start the Sauce: Melt half the butter (6 tbsp) in a large skillet over medium heat until foaming. Add the minced garlic and red pepper flakes. Cook, stirring constantly, for 1-2 minutes until fragrant and light golden. Do not let the garlic burn.
Cook the Shrimp: Pat the shrimp completely dry and season generously with salt and pepper. Increase heat to medium-high. Add shrimp to the skillet in a single layer (work in batches if needed). Cook for 2-3 minutes per side, until pink, opaque, and lightly golden. Do not overcook. Transfer shrimp to a clean plate.
Finish the Sauce: Reduce heat to medium. Add the remaining 6 tbsp of butter, chopped parsley, and fresh lemon juice to the skillet. Stir constantly, scraping up any browned bits, until the butter is melted and the sauce is glossy and emulsified. Taste and adjust seasoning.
Combine & Serve: Return the cooked shrimp to the skillet. If using pasta, add the cooked linguine. Toss everything to coat. If the sauce is too thick, add a splash of the reserved pasta water to loosen it. Serve immediately, garnished with extra parsley, lemon wedges, and crusty bread on the side.