Preheat the oven to 350°F (175°C). Grease a 9x13-inch (3 Qt) baking dish with cooking spray or melted butter and set it aside. In a small bowl, combine the panko breadcrumbs, melted butter, and seasoned salt to make the crunchy topping. Mix well and set aside for later.

Bring a large pot of salted water to a boil. Add the dry macaroni noodles and cook according to package instructions until almost al dente (about 2 minutes less than the time listed on the package). Drain the pasta, then transfer it to a large bowl. Drizzle with a bit of olive oil or butter and toss gently to prevent sticking. I find a small amount goes a long way in keeping the noodles separated until you add the cheese sauce.

In a large saucepan over medium-high heat, melt the unsalted butter (be careful not to let it brown). Sprinkle in the flour and whisk to form a roux, cooking and stirring for about 1 minute. Gradually whisk in the whole milk in a thin, steady stream, then add the half & half, whisking constantly until combined and smooth. Whisk in the ground black pepper, pinch of nutmeg, garlic powder, and dry mustard. Continue to cook and stir until the sauce comes to a simmer and thickens, about 6–8 minutes. You'll know it's ready when the sauce coats the back of a spoon and you can draw a line through it with your finger.

Reduce the heat to low. Working in increments, add the shredded sharp cheddar cheese to the hot sauce, stirring until each addition is fully melted and combined. Next, add the white cheddar cheese gradually, stirring into the mixture until melted, saving a portion for topping the pasta later. Then, add the Gruyere cheese and stir until the sauce becomes smooth and creamy. Remove the sauce from the heat. I like to add the cheese little by little to keep the sauce extra smooth and prevent clumps.

Pour the finished cheese sauce from Step 4 over the cooked macaroni noodles from Step 2. Mix until all the pasta is evenly coated. Stir in any reserved white cheddar cheese and mix well. Taste and adjust seasoning with salt and pepper if desired.

Transfer the cheesy pasta mixture into the prepared baking dish. Smooth the surface into an even layer. Sprinkle the panko breadcrumb topping from Step 1 evenly over the pasta. Bake at 350°F (175°C) for 20 minutes, or until bubbling and set. If the top needs extra browning, place it under the broiler for 2-3 minutes, watching carefully to prevent burning. I always keep a close eye during broiling—the difference between golden and burnt is only a minute!

Remove the mac and cheese from the oven and let it rest for 10 minutes before serving. This rest time helps the sauce thicken and makes the dish easier to serve.