Cook the pasta 1 minute short of al dente according to package directions -- so if "al dente" is 10 minutes, cook for 9. Pour pasta into a colander and rinse with cold water to stop the cooking. Set aside to drain.
In a mini-prep food processor add the bread, butter and parsley and pulse until crumbs form. Set aside.
Spray an 8" casserole dish with vegetable spray and set aside. Preheat the oven to 350°.
In a large saucepan, melt the 3 tablespoons of butter over medium to medium high heat and whisk in the mustard, cayenne pepper and flour. Stir for one minute over the heat then add the lobster stock and continue whisking -- mixture will absorb and thicken quickly. Add the milk and cream and continue to whisk. Heat the mixture just to the boiling point, whisking the whole time until it thickens -- and being careful to scrape the bottom and sides of the pan so that the flour mixture doesn't settle onto the bottom.
Remove the sauce from heat and stir in the pasta. Add the lobster and 1 3/4 cups gruyere cheese and parsley. Stir to combine. Pour the pasta mixture into the prepared dish. Sprinkle with breadcrumbs and remaining gruyere cheese.
Bake for 15 minutes or until breadcrumbs are lightly browned and casserole is bubbly. Jump up and down because it's ready and your about to be a very happy camper!