Sauté onions in butter in a large skillet over medium heat until translucent. Transfer to a separate dish. Add lobster shells, garlic, shallots, and peppercorns to skillet. Cover with water and simmer for 20–30 minutes. Strain liquid and discard shells/onions/garlic.
Place strained liquid in a measuring pitcher. Add water if necessary to obtain 2 quarts .Bring to a boil. Remove from heat. Add gravy mix while whisking vigorously. Let stand for 3–5 minutes and whisk again.
Add cheeses to gravy and stir until melted and smooth. Season to taste with salt and pepper.
Place lobster, cooked pasta, and onions in a large bowl. Pour gravy mixture over pasta and stir to evenly coat all pieces.
Pour into 2 hotel pans sprayed with pan release. Top with panko breadcrumbs. Bake at 350˚F for 25–30 minutes (325˚F convection for 20–25 minutes).