Boil the pasta: Bring a large pot of well-salted water to a rolling boil. Add spaghetti and cook until just shy of al dente according to package directions. Before draining, reserve 1–1.5 cups of pasta water.

Prep aromatics: While pasta cooks, finely mince the garlic. Zest the lemon first, then juice it. Have everything ready—this dish moves quickly.

Melt the butter: In a large skillet over medium heat, melt the butter. Add olive oil if using to keep the butter from browning too quickly and to add a little body.

Gently cook the garlic: Add garlic to the melted butter and cook 60–90 seconds until fragrant and just turning pale golden. Do not let it brown, or it will taste bitter. Add a pinch of red pepper flakes and a grind of black pepper.

Add lemon: Stir in the lemon zest and 2 tablespoons lemon juice. Swirl to combine. The mixture should smell bright and buttery.

Build the sauce: Add a generous splash of reserved pasta water (about 1/2 cup) and bring to a gentle simmer. This helps the butter and lemon emulsify into a silky sauce.

Toss in the pasta: Add the drained spaghetti to the skillet. Toss continuously, adding more pasta water as needed to create a glossy coating. Taste and adjust salt, lemon juice, and pepper.

Add cheese off heat: Remove the pan from heat. Sprinkle in the Parmesan and toss until the sauce is creamy. If it’s too thick, loosen with another splash of pasta water.

Finish: Stir in chopped parsley. Serve immediately with extra Parmesan, more pepper, and a light squeeze of lemon if you like.