Preheat the oven to 400 if you are baking your mac and cheese. If you're just broiling the top, you can skip this step.
If using breadcrumbs, pulse 2 or 3 slices of fresh bread in the food processor until crumbs form. Add melted butter and stir to combine. Set aside.
Bring a large pot (about 3 quarts) of water to a boil and add 1 tablespoon of salt. Add the pasta and set a timer for two minutes less than the box suggests. If it gives a range, use the shortest time.
Once the pasta is in the pot, start the sauce:In a 12-inch skillet, melt the butter over medium high heat. Add the flour and stir to cook out the raw flour flavor--about 1 minute. Add the milk and stir to combine. Add salt and pepper and reduce heat to medium low.
Cook until the sauce simmers, stirring constantly, and the sauce is thick enough to coat the back of a spoon--about 5-6 minutes. Don't walk away from the sauce or it might split!
Turn off the heat and add the cheese. Stir until the cheese melts then add the Old Bay, lobster, parsley, vinegar, and nutmeg. Add the pasta and stir to combine.
Transfer to a greased 8x8 baking dish or bake it in your skillet if it’s oven safe. Top with breadcrumbs and bake 15-20 minutes on the center rack until bubbling on the edges and the cheese is brown in spots.
Broil only option: If you want to broil the top, preheat the broiler and broil--about 5 inches from the element or flame--5 minutes or so. Keep an eye on it so you don't burn it!