Bring a large pot of water to a boil and salt it generously, it should taste like seawater.
Add spaghetti and cook 1 minute less than the package time, until al dente with a firm bite at the center. Reserve 1 cup of pasta water, then drain.
While the pasta cooks, heat olive oil in a large skillet over medium-low heat. Add sliced garlic and cook 2-3 minutes, stirring often, until pale gold at the edges.
Add chile flakes and stir 30 seconds until fragrant, watching that the garlic doesn't darken further.
Add the drained spaghetti to the skillet with a splash of reserved pasta water, about 60 ml. Toss over medium heat 1-2 minutes, adding more water as needed, until the sauce clings and the pasta looks glossy.
Remove from heat, stir in parsley and black pepper, and taste for salt.
Serve immediately in warmed bowls with an extra drizzle of olive oil.