Preheat oven to 375°F and grease a 9x13 baking dish (or two 8x8-inch baking dishes).

Cook spaghetti in boiling water for 6 minutes. Drain and set aside.

In a large saucepan, melt butter over medium heat. Whisk in flour and cook for 30 seconds, stirring constantly.

Whisk in salt, black pepper, granulated garlic, and cayenne pepper. Gradually whisk in the half-and-half and heavy cream.

Bring the mixture to a boil, then reduce to a simmer. Stir in half of the mozzarella and cheddar cheese until smooth.

Add cooked spaghetti to the cheese sauce and mix until fully coated.

Pour the cheesy spaghetti into the prepared baking dish and top with the remaining cheeses.

Bake for 25–30 minutes, or until the top is golden and the edges are bubbling. Let rest before serving.