In a heavy 6-quart saucepan over low heat, melt butter and add onions and garlic. Slowly sweat over low heat until translucent, about 5 minutes. Add flour, stirring to combine, and incorporate all dry ingredients, along with hot sauce. Stir until uniform. Slowly whisk in heavy cream. Raise heat to medium, add cheeses and stir constantly just until cheese is melted. Cook's Note: do not bring this mixture to a boil; the temperature shouldn't exceed 140 degrees F.
Bring 2 gallons of salted water (you should be able to taste the salt, about 1/2 cup) to a boil in a 12-quart pot. Add macaroni; boil about 6 minutes, or until al dente. Strain, and immediately cool by rinsing with ice-cold water.
Ten minutes before you want the mac on the table, in a very large nonstick pan, warm cheese sauce over medium-high heat, stirring constantly with a heat-resistant rubber spatula. Once cheese sauce comes to a simmer, add cold pasta and lobster meat. Return to a simmer, stirring constantly. Turn out warm noodles and sauce into large casserole dish, about 6-quart size. Cook's Note: For individual servings, separate noodles into 8 individual casserole dishes.
Top generously with additional shredded Cheddar and bread crumb mixture. Smooth bread crumbs into an even layer over the pasta and pat down with fingers to form a loose crust. Place under broiler until crumbs are an even golden brown. Garnish with chopped parsley, and serve immediately.
In a food processor, combine parsley, garlic, oil and salt. Split lobsters in half from the head down. Remove claws and cook for 5 minutes in boiling water. Remove parcooked claw and knuckle meat and marinate along with split tails in parsley mixture for 20 to 30 minutes. On a well oiled high heat grill, grill lobster tail meat down, along with claw and knuckle meat until charred slightly. Chop lobster meat into 1/2-inch cubes.
In a medium bowl, mix all ingredients until combined. Set aside.