Bring a large pot of well-salted water to a rolling boil. Use 1 tablespoon salt per 4 quarts water.

Add spaghetti and cook until 1 minute shy of al dente (about 1 minute less than package directions). Stir occasionally.

While pasta cooks, heat olive oil in a 12-inch skillet over medium-low heat. Add sliced garlic and red pepper flakes. Cook slowly, stirring often, until garlic is fragrant and just golden—about 3-4 minutes. Do not let it brown or burn. If it darkens too fast, remove pan from heat briefly.

When pasta is done, reserve 1 cup of pasta water, then drain spaghetti.

Add drained spaghetti to the skillet with the garlic oil. Increase heat to medium. Toss with tongs for 1 minute, adding pasta water a few tablespoons at a time, until the sauce emulsifies and coats each strand. You'll likely use about 1/2 cup total. The pasta should look glossy, not greasy.

Remove from heat. Toss in parsley. Taste and add salt if needed (the pasta water usually provides enough). Serve immediately with Parmesan on the side.