Cook the pasta: Bring a large pot of well-salted water to a boil. Add 12 ounces spaghetti and cook it 2 minutes less than the package directions. Before draining, reserve 2 cups of the pasta water, then drain the pasta.
Make the lemon sauce: While the pasta cooks, warm 4 tablespoons extra virgin olive oil, 2 cloves garlic (grated), ¼ teaspoon red pepper flakes, and the grated zest of 1 large lemon in a large skillet over low heat for 30 to 60 seconds, stirring often. Don't let the garlic brown. Turn off the heat and stir in the lemon juice.
Toss the pasta: Add the drained pasta to the skillet and set it over medium heat. Pour in 1 cup reserved pasta water and toss for 1 to 2 minutes, until the pasta is al dente and coated in a silky sauce.
Finish and serve: Turn off the heat and stir in ½ cup fresh flat-leaf parsley and ½ cup grated Parmigiano Reggiano until the cheese melts. Add more pasta water if needed to loosen the sauce. Taste, season with salt and black pepper to taste, and serve right away with extra parmesan and lemon zest, if you like.
Substitutions
Spaghetti → Linguine, fettuccine, bucatini, or angel hair.
Extra virgin olive oil → Mild regular olive oil (extra virgin gives the best flavor).
Garlic → Omit it if you're garlic intolerant, or infuse the oil with a whole garlic clove and remove it before serving.
Red pepper flakes → Finely chopped dried red chili, Calabrian chili flakes, or freshly ground black pepper for a milder flavor.
Lemon → Meyer lemons for a slightly sweeter, less tangy flavor.
Parsley → Chives or fresh basil.
Parmigiano Reggiano → Grana Padano or Pecorino Romano.
Pasta water → No substitute—it's the key to a silky sauce without cream.
Easy Add-Ins
Tuna: Warm a can of tuna in olive oil in the sauce before adding the pasta.
Chicken or shrimp: Top with grilled chicken, rotisserie chicken, or sautéed shrimp.
Cannellini beans: Stir in a can during the last minute of cooking.
Chickpeas: Add a can of drained chickpeas or crispy roasted chickpeas.
Spinach or arugula: Toss in just before serving to wilt.
Peas or asparagus: Add frozen peas or blanched asparagus for a fresh twist.
Tips
Salt the water well: About 2 tablespoons kosher salt (or 1 tablespoon fine sea salt) per large pot.
Cook pasta 2 minutes less: Finish it in the skillet for perfect al dente pasta.
Use good olive oil: With so few ingredients, quality makes a big difference.
Zest before juicing: It's easier and gives the brightest lemon flavor.
Keep the heat low: Don't let the garlic brown or it can turn bitter.
Save the pasta water: It's the secret to a silky sauce without cream.
Add parmesan off the heat: It melts smoothly and won't clump.
Serve right away: Finish with extra lemon zest for the freshest flavor.
Store leftovers: Refrigerate in an airtight container for up to 3 days. Reheat gently with a splash of water.