Boil a large pot of heavily salted water. Cook the spaghetti for about 8 minutes, until just shy of al dente. Reserve 1 cup of pasta water, then drain.

While the pasta cooks, warm the olive oil in a large skillet over medium-low heat. Add the sliced garlic and red pepper flakes. Sizzle gently for 3 to 4 minutes, until the garlic is pale gold.

Add the drained spaghetti to the skillet. Toss with the garlic oil, pour in about ⅓ cup of the reserved pasta water, and keep tossing for 1 to 2 minutes until a glossy sauce forms. Off the heat, fold in the parsley, season with salt, and serve immediately.