Bring a large pot of well-salted water to a boil. Add the short pasta and cook until just al dente, about 1 minute less than the package instructions. Drain, then toss with 1 tablespoon butter to prevent sticking and set aside.
1 pound short pasta, 1 tablespoon unsalted butter
If not already done, chop the cooked lobster meat into bite-size pieces. If it was frozen, make sure it is fully thawed and pat very dry with paper towels. Toss with the lemon juice and set aside.
1 1/2–2 cups cooked lobster meat, 1 tablespoon fresh lemon juice
In a saucepan or in the microwave, gently warm the milk until hot to the touch but not boiling. Keep warm while you make the roux.
4 cups whole milk
In a large heavy-bottomed pot or Dutch oven, melt 6 tablespoons butter over medium heat. Add the flour and whisk constantly for 1–2 minutes until it forms a smooth paste and smells slightly nutty but is not deeply browned.
6 tablespoons unsalted butter, 6 tablespoons all-purpose flour
Slowly whisk in the warm milk, about 1 cup at a time, whisking well after each addition to keep the mixture smooth. Once all the milk is added, continue cooking, whisking often, for 5–7 minutes, until the sauce thickens slightly and coats the back of a spoon.
4 cups whole milk
Stir in the kosher salt, black pepper, smoked paprika, dry mustard powder, garlic powder, onion powder, and Old Bay if using. Taste and adjust seasoning; the sauce should taste slightly more seasoned than you want the finished dish, since the pasta will mellow it.
1 teaspoon kosher salt, 1/2 teaspoon freshly ground black pepper, 1/2 teaspoon smoked paprika, 1/2 teaspoon dry mustard powder, 1/4 teaspoon garlic powder, 1/4 teaspoon onion powder, 1–2 teaspoons Old Bay seasoning
Reduce the heat to low. Add the grated cheddar and Gruyère in small handfuls, stirring well after each addition until completely melted and smooth. Stir in the Parmesan last. If the sauce looks like it may separate, turn off the heat and keep stirring until it is creamy and glossy.
3 cups sharp cheddar cheese, 1 1/2 cups Gruyère cheese, 1/2 cup Parmesan cheese
Add the cooked pasta to the cheese sauce and gently fold until evenly coated. Then add the lobster pieces and carefully fold them in, trying not to break them up too much. If the mixture seems overly thick, stir in 2–4 tablespoons additional warm milk until creamy and pourable.
1 pound short pasta, 1 1/2–2 cups cooked lobster meat, 4 cups whole milk
Preheat the oven to 350°F (175°C). Lightly grease a 9×13-inch (about 3-quart) baking dish. Transfer the lobster mac and cheese mixture to the dish and smooth the top.
In a small bowl, combine the panko breadcrumbs, melted butter, Parmesan, and chopped parsley if using. Toss until the crumbs are evenly coated, then sprinkle the mixture evenly over the top of the pasta in the baking dish.
1 cup panko breadcrumbs, 2 tablespoons unsalted butter, 2 tablespoons Parmesan cheese, 1–2 tablespoons fresh parsley
Bake at 350°F (175°C) for 20–25 minutes, or until the edges are bubbling and the topping is lightly golden. For an extra-crispy top, switch the oven to broil for 1–3 minutes at the end, watching closely so the crumbs do not burn.
Let the lobster mac and cheese rest for 5–10 minutes before serving so it can set slightly. Garnish with additional parsley or a light sprinkle of paprika if desired, then serve warm.
1–2 tablespoons fresh parsley, 1/2 teaspoon smoked paprika