Season the filet with salt and pepper, to taste, and garlic powder on both sides. Combine the remaining ingredients in a zip-top bag. Add the meat to the bag and marinate in the refrigerator for 2 to 4 hours.
Preheat oven to 400 degrees F.
Transfer filet to a broiler pan and roast in the oven for 12 to 20 minutes, depending on desired doneness. Remove from oven and let rest before slicing.
Remove the raw lobster meat from the shell and cut into chunks.
In a large skillet over medium-high heat, melt butter and saute onions until softened. Add the lobster meat and cook until it just begins to turn pink. Then stir in lemon juice and cook until it has reduced by half. Then stir in the cream and cook and stir until cream thickens. Stir in the Parmesan, parsley and salt and pepper, to taste.
Build your sandwich starting with some of the sliced filet mignon on the toasted roll. Spoon some of the lobster chunks on top of the steak. Load on the french fries. Top with a spoonful of the lobster's Parmesan sauce. Pile on some lettuce, tomato and onion slices, if desired.