Bring a large pot of salted water to a boil and cook the spaghetti according to package directions until al dente.
Reserve a small cup of pasta cooking water, then drain the spaghetti in a colander and set aside.
Heat a skillet over medium heat and add the olive oil and melted butter until shimmering.
Add the minced garlic and sauté until fragrant, about 30–60 seconds, taking care not to brown it.
Toss the drained spaghetti into the skillet and season with salt, ground black pepper, and red chili flakes.
Toss or stir to combine, adding a splash of reserved pasta water if needed to loosen the sauce; cook 1–2 minutes to meld flavors.
Transfer to plates, garnish with finely chopped parsley, and serve immediately.