Preheat your oven to 350 degrees .
Melt 1 tablespoon of the butter in a large skillet over medium high heat. Add shrimp and cook, stirring constantly, just until pink (about 3 minutes). Remove from heat and set aside.
In a large saucepan, melt the remaining 6 tablespoons of butter. Transfer 2 tablespoons of the melted butter to a medium bowl and toss with the panko bread crumbs.
Add diced onions and garlic to melted butter in saucepan and cook until softened, about 3-4 minutes, stirring constantly.
Add flour and cook for an additional minute, stirring constantly.
Add milk and whisk until smooth and there are no lumps.
Bring to a gentle boil, reduce heat to maintain a simmer and cook until thickened, whisking constantly.
Season with salt, pepper, nutmeg and Old Bay Seasoning.
Remove saucepan from burner. Add cheeses and stir until smooth. Fold in macaroni, shrimp and crab meat.
Transfer to an 2.5 qt casserole dish that has been sprayed with cooking spray.
Top evenly with buttery panko bread crumb mixture.
Bake for 20-30 minutes, or until bread crumb topping is golden brown and sauce is bubbling around edges.