Preheat oven to 400 degrees Fahrenheit with rack in the middle of oven. Melt 1/2 cup unsalted butter in a large saucepan over medium heat. Add 6 tablespoons all-purpose flour to melted butter and stir until thickened, about 3 minutes.: The oven warming creates an even environment so the top browns uniformly, and placing the rack in the middle prevents the top from burning while the interior heats. As it warms you might notice the kitchen becoming gently warm and the oven light glowing, which is a cue it is getting close. Avoid placing the dish too close to the heat source, or you risk an overly crisp top while the middle remains cool. A common oversight is not waiting for a full preheat, which can lengthen bake time and affect texture.
Slowly whisk in 5 cups whole milk a little at a time. The slower you add the milk the thicker the sauce will be. Stir in 2 cups grated cheddar cheese and 1/2 cup grated parmesan cheese into the sauce.: As the butter melts it will foam then settle, releasing a slightly nutty aroma when it begins to brown, though you should keep it pale for this sauce. This step prepares the fat for the roux, which is essential for thickening. Watch closely, because if the butter browns too much it will impart a toasty flavor that can be distracting. A frequent mistake is using too high heat, which can cause the butter to separate.
Add 2 teaspoons salt, pepper to taste, 1 teaspoon garlic powder, 1 teaspoon mustard powder, and 1/2 teaspoon nutmeg to the sauce, stir until combined. Stir in 1 pound cooked elbow macaroni into the sauce and transfer to a 9 x 13 inch baking sheet.: You will notice the mixture become pasty and smooth, and the raw flour scent will dissipate as it cooks, leaving a slightly toasted smell. This forms the roux that thickens the sauce and prevents graininess. Stir continuously to cook the flour evenly, because undercooked flour tastes chalky. Many people stop too soon, so count gently to ensure about 3 minutes of gentle cooking.
Sprinkle the top with 11/2 cups shredded mozzarella cheese, 1/2 cup parmesan cheese and bake in the oven until golden and bubbling, 20-25 minutes. Sprinkle with chopped flat leaf parsley for garnish.: Adding milk gradually creates a silky béchamel, and the whisking helps break up lumps so the sauce becomes smooth. You will see the sauce shift from thick paste to glossy liquid, and steam will rise as it warms. Pouring the milk too quickly can create lumps, while adding it slowly yields a silkier texture. If lumps form, whisk vigorously and cook a bit longer to smooth them out.
Stir until thickened, about 3 minutes: As the sauce thickens you will notice it coat the back of a spoon and slow when stirred, creating a velvety sheen. This thickness is crucial so the sauce clings to the macaroni rather than pooling. Keep the heat moderate so the sauce does not boil violently, which can separate it. Overshooting the thickening can make it pasty, so remove from heat when it naturally coats the spoon.
Stir in 2 cups grated cheddar cheese and 1/2 cup grated parmesan cheese into the sauce: The cheeses will melt into the warm béchamel, producing a glossy, cohesive sauce that smells rich and savory. Stirring gently encourages even melting and prevents the cheddar from becoming stringy or the parmesan from clumping. If the sauce seems grainy, lower the heat and stir slowly to integrate the cheeses. Overheating while the cheese melts can cause it to separate.
Add 2 teaspoons salt, pepper to taste, 1 teaspoon garlic powder, 1 teaspoon mustard powder, and 1/2 teaspoon nutmeg to the sauce, stir until combined: The aromatics and seasonings bloom in the warm sauce, creating layers of flavor where the garlic powder and mustard powder lift the cheese, and the nutmeg adds warmth. Taste and adjust, because cheeses vary in saltiness. A common error is over seasoning early, so add gradually and taste as you go.
Stir in 1 pound cooked elbow macaroni into the sauce and transfer to a 9 x 13 inch baking sheet: The pasta should be just under al dente so it finishes perfectly in the oven; when mixed you will see it evenly coated and glossy. Spreading it into the baking dish ensures even baking and a uniform top crust. If the pasta is overcooked it will fall apart during baking, so drain and shock less if needed. Crowding the pan can lead to uneven browning.
Sprinkle the top with 11/2 cups shredded mozzarella cheese, 1/2 cup parmesan cheese and bake in the oven until golden and bubbling, 20 to 25 minutes: As it bakes the top will transform, bubbling and forming spots of deep golden brown, and you will hear a faint sizzle as moisture escapes. The aroma will intensify and a lightly crisp crust will develop, contrasting with the creamy interior. Keep an eye on color near the end, because ovens vary and you do not want excessive browning. Opening the oven frequently can lengthen bake time, so check visually through the window when possible.
Sprinkle with chopped flat leaf parsley for garnish: The fresh parsley adds a bright herbal note and a pop of color against the golden top, making the dish look lively. Scatter it right after the bake so it stays vibrant. Adding it before baking will wilt it and mute the color, which reduces visual appeal.