1-First Step: Get the oven and pasta water ready
Arrange the oven racks so one sits in the middle and one near the top. Heat the oven to 375°F. While the oven warms, bring a large pot of heavily salted water to a boil. Salting the water well is important because it seasons the spaghetti from the inside out.
Add 1 pound dried spaghetti noodles and cook them for 1 minute less than al dente according to the package directions. This keeps the pasta firm enough to finish cooking in the oven without turning soft. When the pasta is ready, drain it and set it aside briefly.

2-Second Step: Start the Alfredo sauce
Set a large pot over medium heat and melt 3 tablespoons unsalted butter. Once the butter is melted and foamy, whisk in 3 tablespoons all-purpose flour. Cook this mixture for a short time until it turns lightly golden. That step takes away the raw flour taste and gives the sauce a smoother flavor.
Keep whisking so the mixture does not clump or scorch. You are building the base that will thicken the milk and cream later on.

3-Third Step: Add the milk and cream slowly
Slowly whisk in 3 cups milk and 1 cup heavy cream. Add the liquid in a steady stream and keep whisking while you go. This helps the sauce stay smooth instead of lumpy.
Bring the mixture to a boil, then reduce the heat and let it simmer until it thickens. You want a sauce that coats a spoon, not one that runs thin. If you are new to white sauces, this is the moment to stay close and keep stirring.

4-Fourth Step: Stir in the cheese and seasonings
Take the pot off the heat. Whisk in 4 ounces of the shredded mozzarella, 3 ounces of the grated Parmesan, 1/2 teaspoon garlic powder, and 1 teaspoon kosher salt. The sauce should turn glossy and smooth as the cheese melts.
Taste the sauce if you like, then adjust with a little more salt only if needed. Since Parmesan is naturally salty, a small amount of seasoning often goes a long way. Freshly grated cheese makes the sauce taste better and melt more evenly than shelf-stable pre-shredded versions.

5-Fifth Step: Combine the pasta and sauce
Add the drained spaghetti to the sauce and toss until every strand is coated. Use tongs if you have them, because they make it easier to mix the noodles evenly. You want the sauce to nestle into the pasta, not pool at the bottom of the pan.
If the sauce seems very thick, you can add a small splash of milk to loosen it a little before baking. The pasta should look generously coated and ready for the oven.

6-Sixth Step: Assemble the casserole
Transfer the sauced spaghetti into a baking dish. Spread it into an even layer so it bakes uniformly. Sprinkle the remaining mozzarella and Parmesan over the top. That final layer creates the golden, bubbly finish everyone expects from a baked pasta dish.
If you want a little extra browning, keep some cheese a bit heavier in the center and around the edges. That gives the top a nice mix of melty spots and crisp bits.

7-Seventh Step: Bake until bubbly
Place the dish on the middle rack and bake for 20 minutes. The casserole should become hot all the way through and start bubbling around the edges. If the top needs a little more color, move it to the top rack and broil for 3 to 4 minutes.
Watch closely during broiling because cheese can go from golden to burnt very quickly. A light brown top and bubbling edges are what you are looking for.

8-Eighth Step: Rest and serve
Take the casserole out of the oven and let it cool for 5 minutes before serving. This short rest helps the sauce settle so the portions hold together better on the plate. Finish with finely chopped parsley leaves if you want a fresh pop of color.