Gather all ingredients.

Bring a large pot of lightly salted water to a boil. Cook elbow macaroni until tender yet firm, about 8 minutes. Reserve 2 cups pasta water. Drain and rinse macaroni with cold water.

Pour reserved pasta water into the same pot, add lobster halves cut-side up, and steam until lobster meat firms and turns opaque, about 3 minutes.

Remove lobster, cool briefly, then cut lobster meat into bite-sized pieces and reserve shells.

Melt 2 tablespoons butter in a saucepan, cook onion until softened and translucent, about 5 minutes. Remove onions to a bowl and set aside.

Place reserved lobster shells, milk, shallot, peppercorns, and garlic into the same saucepan. Simmer gently for 20 minutes.

Preheat the oven to 350°F (175°C).

Melt remaining 5 tablespoons butter, whisk in flour to form a paste, and cook until golden, about 10 minutes. Strain milk mixture and slowly pour into flour while whisking. Simmer until thick and smooth, 10 to 15 minutes.

Stir Gruyère, Cheddar, and Romano cheeses into milk mixture until melted and smooth. Season with salt and pepper. Add lobster meat, onions, and macaroni.

Pour mixture into a 4-quart casserole dish, smooth the top, and sprinkle evenly with panko crumbs.