Fill a large pot with water, add a generous pinch of salt, and bring to a boil.

Add the spaghetti and cook according to package directions, about 9-10 minutes, until al dente. Reserve 1/3 cup of pasta water, then drain the spaghetti using a colander.

In a medium saucepan, melt the butter over medium heat. Add the flour and whisk constantly for 1-2 minutes to make a roux; don’t let it brown.

Gradually pour in the milk, whisking continuously to avoid lumps. Cook for 3-4 minutes until the sauce begins to thicken.

Lower the heat and add the cheddar, mozzarella, and parmesan cheeses to the sauce, stirring until melted and smooth.

Stir in the garlic powder, salt, and black pepper. If the sauce is too thick, add a bit of the reserved pasta water to reach your desired consistency.

Add the drained spaghetti to the cheese sauce, tossing with tongs or a wooden spoon to coat the pasta evenly.

(Optional) For a baked version: Transfer the cheesy spaghetti to a greased oven-safe baking dish, sprinkle with extra cheese, and bake at 200°C (400°F) for 10-15 minutes until golden and bubbly.

Garnish with chopped fresh parsley before serving.