Preheat the oven to 375 degrees F. Spray a 4-quart casserole dish with cooking spray and set aside.
Cook the pasta in salted water according to the directions on the package, to al dente. Drain and set aside.
Meanwhile, melt 6 tablespoons of butter in a medium saucepan over medium heat. Stir in the onion and cook for 4-5 minutes until soft and translucent. Add garlic and saute for 30 seconds, until fragrant. Sprinkle in the flour and cook over low heat for 2 minutes, whisking constantly, until golden. Whisk in the broth, scraping any brown bits from the bottom of the pan.
Still whisking, pour in the milk, bring to a gentle simmer over medium heat, cooking until thickened and smooth.
Remove the pan from the heat and add the Gruyere, cheddar, all but 2T of the Parmesan, salt, pepper, and nutmeg, stirring until melted and combined.
Add the cooked macaroni and lobster and stir well.
Transfer the mixture to the prepared casserole dish.
Melt the remaining butter and stir it into the breadcrumbs and remaining Parmesan, then sprinkle on the top.
Bake for 15-20 minutes, or until the sauce is bubbly and the macaroni is golden brown. Garnish with chopped parsley and serve while hot.