1-First Step: Get the oven and pasta water ready Position one oven rack in the middle and another rack at the top. Preheat the oven to 375°F. Then bring a large pot of salted water to a boil. The salted water seasons the pasta from the inside out, so do not skip that part. Once the water is boiling, add the spaghetti and cook it for 1 minute less than the package directions for al dente. This matters because the pasta will keep cooking in the oven later. Undercooking it slightly helps you avoid mushy baked spaghetti.

2-Second Step: Start the Alfredo-style sauce While the pasta cooks, melt the butter in a pot over medium heat. Whisk in the flour and keep stirring until the mixture turns golden brown, which usually takes about 2 minutes. This step cooks out the raw flour taste and gives the sauce a smoother base. Next, slowly whisk in the milk and heavy cream. Add them gradually so the mixture stays smooth and does not clump. Bring it to a boil, then lower the heat and simmer for about 5 minutes until the sauce thickens enough to coat a spoon.

3-Third Step: Add the cheese and seasoning Take the sauce off the heat. Whisk in most of the mozzarella and Parmesan, keeping a little of each cheese aside for the top. Stir in the garlic powder and kosher salt. The sauce should look glossy, thick, and very cheesy.

4-Fourth Step: Combine the pasta and sauce Drain the spaghetti well, then add it directly to the sauce pot. Toss until every strand is coated. This is the part where the dish really comes together, because the pasta absorbs the creamy sauce and picks up all that flavor. Transfer the coated spaghetti to a baking dish and spread it out evenly. Make sure the noodles are tucked into the sauce rather than sitting dry on top.

5-Fifth Step: Add the topping and bake Sprinkle the remaining mozzarella and Parmesan over the pasta. Bake on the middle rack for about 20 minutes, or until the sauce is bubbling around the edges. Then move the dish to the top rack and broil for 3 to 4 minutes so the cheese browns nicely. Watch closely during broiling. Cheese can go from golden to burnt fast, so stay near the oven and pull it out once the top looks spotty and crisp.

6-Final Step: Rest, garnish, and serve Let the baked spaghetti cool for about 5 minutes before serving. This short rest gives the sauce time to settle, making it easier to scoop. If you want a fresh finish, top it with finely chopped parsley leaves. Serve it warm and creamy while the cheese is still soft and stretchy. It pairs well with a simple salad, garlic bread, or roasted vegetables if you want to round out the meal.