Easy Baked Macaroni & Cheese

1. 10 tablespoons unsalted butter (1 1/4 sticks)
2. 1 cup panko
3. 1 medium yellow onion, small dice
4. 4 teaspoons kosher salt, plus more as needed
5. 1/2 teaspoon freshly ground black pepper, plus more as needed
6. 1/2 cup all-purpose flour
7. 5 cups whole milk
8. 1 pound elbow macaroni
9. 5 ounces shredded sharp cheddar cheese (about 2 1/2 cups)
10. 5 ounces shredded Gouda cheese (about 2 1/2 cups)
11. 2 ounces finely grated Parmesan cheese (about 1 cup)

Instructions

1. Bring a large pot of heavily salted water to a boil over high heat. Heat the oven to 400°F and arrange a rack in the middle. Meanwhile, melt the butter over medium heat in a large saucepan or Dutch oven, then turn off the heat. Transfer 2 tablespoons of the melted butter to a medium bowl, add the panko, season with salt and pepper, and mix with your fingers until evenly combined. Set aside.
2. Return the saucepan to medium heat, add the onion and measured salt and pepper, and cook until softened but not browned, about 6 minutes.
3. Add the flour and whisk frequently until the mixture turns light brown in color, about 5 to 6 minutes.
4. While whisking, slowly add the milk to the flour mixture until evenly combined and smooth, stopping to whisk any clumps as needed. (It will get very thick when you first add the milk.)
5. Continue cooking, whisking occasionally, until the sauce thickens and coats the back of a spoon, about 7 minutes. Remove from the heat and set aside. Meanwhile, add the pasta to the boiling pot of water and cook until still chewy and underdone, usually a few minutes less than the package directions suggest (the pasta will finish cooking in the oven). Drain and rinse with cold water until no longer steaming; set aside.
6. Return the milk mixture to medium heat and stir in the cheeses until melted and smooth. Add the rinsed pasta and continue cooking, stirring occasionally, until the pasta is heated through and steaming, about 4 to 5 minutes. Taste and season with salt and pepper if needed.
7. Transfer to a 5-quart baking dish, sprinkle with the reserved panko mixture, and bake until bubbling and lightly browned on top, about 25 to 30 minutes. Transfer to a wire rack and let cool for 5 minutes before serving.