Bring a large pot of water to a rolling boil. Add 1 tablespoon kosher salt. Add the pasta shells and cook until just shy of al dente (about 9–10 minutes, check package). Reserve 1/2 cup of the pasta cooking water, then drain shells and return to the pot. Toss with 2 tablespoons unsalted butter to prevent sticking and set aside.

Prepare lobster

If using pre-cooked lobster, check for any cartilage and chop meat into 1/2–3/4-inch pieces. If using raw lobster tails, poach or steam until opaque (about 6–8 minutes), cool slightly, then remove meat and chop. Keep lobster chilled until ready to fold into the sauce.

Cheese Sauce

Make roux

In a medium saucepan over medium heat, melt 3 tablespoons unsalted butter. Add 3 tablespoons flour and whisk continuously for 1–2 minutes until the mixture is bubbling and slightly nutty but not browned.

Add dairy

Slowly whisk in 2 cups whole milk and 1 cup heavy cream in a steady stream to prevent lumps. Continue whisking and bring the mixture to a gentle simmer; it will thicken in 3–5 minutes.

Season sauce

Stir in 1 teaspoon Dijon mustard, 1 teaspoon Worcestershire sauce, 1 teaspoon cayenne pepper (adjust to heat preference), 1 teaspoon sea salt, and 1 teaspoon freshly ground black pepper. Taste and adjust seasoning as needed.

Add cheeses

Remove the sauce from the heat. Gradually add 2 cups grated sharp cheddar, 1 cup grated Gruyère, and 1 cup finely grated Parmesan, stirring until smooth and fully melted. If the sauce is too thick, whisk in up to the reserved 1/2 cup pasta water, 1 tablespoon at a time, until a creamy consistency that will coat the pasta is reached.

Combine pasta and sauce

Add the butter-tossed pasta shells to the cheese sauce and fold gently to coat. Stir in the chopped lobster meat and fold just until evenly distributed; avoid over-stirring to keep lobster pieces intact. If the mixture seems tight, add a splash of milk or reserved pasta water to loosen (1–2 tablespoons).

Topping & Bake

Prepare topping

In a small bowl combine 3 tablespoons panko breadcrumbs, 2 tablespoons melted butter, 1 tablespoon chopped parsley, and 1 teaspoon lemon zest. Mix until crumbs are evenly moistened.

Assemble

Preheat the oven to 375°F (190°C). Transfer the lobster mac and cheese into a lightly buttered 8x8-inch or similar baking dish, smoothing the top. Evenly sprinkle the panko topping over the surface. Reserve a tablespoon of Parmesan to grate over the top if desired.

Bake

Bake in the preheated oven for 15–18 minutes until the sauce is bubbly and the topping is golden. For a crisper top, place under the broiler for 1–2 minutes while watching closely to prevent burning.

Finish & serve

Remove from oven and let rest 5 minutes. Garnish with an extra pinch of chopped parsley and a light squeeze of lemon if desired. Serve hot.