Preheat the oven to 400°F. Coat a 2-quart baking dish with cooking spray, set aside.
In a large saucepan, cook pasta according to package directions for 1 minute less than it instructs for al dente. Drain and set aside while cooking cheese sauce.
In a medium saucepan, melt the butter over medium heat. Add the onion and garlic, sauteing for 2-3 minutes or until just softened and fragrant.
Whisk in the flour until it forms a paste. Slowly whisk in the milk, nutmeg and seafood seasoning and bring to a low simmer. When bubbles start to form, remove from the heat.
Off heat, whisk in 2 cups of the cheddar cheese and ¾ cup of the Parmesan cheese. Toss the cheese sauce with the cooked pasta and seafood. Transfer to baking dish. Set aside.
In a small skillet, heat the olive oil over medium heat. Add the panko, seafood seasoning and white pepper, stirring until it is combined and lightly toasted. Panko should still be fairly dry and crumbly, not pasty.
Sprinkle the remaining 1 cup cheddar and ¼ cup Parmesan, then the panko topping over mac and cheese. Transfer the dish to the heated oven. Bake, uncovered, for 20 minutes or until the edges are browned and bubbly.
Remove and allow to rest for 5-10 minutes before serving. Top with snipped chives or parsley for garnish.
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*You can use a lower fat content milk, but the sauce might be thinner