1) Heat the oven and prep the dish
Preheat oven to 375 F. Butter a 9x13-inch baking dish.
2) Cook the pasta
Bring a large pot of salted water to a boil. Cook pasta 1 to 2 minutes shy of al dente. Drain, then toss with a small splash of milk or a tiny bit of butter to prevent sticking.
3) Make the béchamel
In a medium saucepan over medium heat, melt 6 tablespoons butter. Whisk in flour and cook 1 to 2 minutes until it smells nutty and looks like a smooth paste.
Slowly whisk in warm milk, a little at a time at first, until smooth. Add cream. Simmer 3 to 5 minutes, whisking often, until thick enough to coat the back of a spoon.
4) Turn it into cheese sauce
Turn heat to low. Stir in Dijon, garlic powder, paprika and cayenne if using, salt, and black pepper.
Remove the pot from the heat. Add cheddar and Gruyere in handfuls, stirring until melted. Stir in Parmesan. Taste and adjust salt and pepper.
5) Fold in lobster and pasta
In a bowl, toss lobster with lemon zest and lemon juice. Stir lobster into the cheese sauce, then fold in the cooked pasta until everything is coated.
6) Add to the baking dish
Spoon the mixture into the prepared dish and spread evenly. If you want extra crisp edges, use a slightly larger or metal baking dish.
7) Top it
Skillet method (extra crisp): In a small skillet over medium heat, toast the panko or crushed crackers for 2 to 4 minutes, stirring often, until lightly golden and fragrant. Slide off the heat, then stir in the melted butter and parsley if using.
Quick method: Skip the skillet and just mix the panko or crushed crackers with melted butter and parsley if using.
Sprinkle the topping evenly over the mac.
8) Bake until bubbly
Bake 20 to 25 minutes until bubbling around the edges and golden on top.
Optional: Broil 1 to 2 minutes for deeper browning. Stay close because it can go from perfect to scorched fast.
9) Rest, then serve
Let rest 10 minutes so the sauce sets slightly. Finish with chives, black pepper, or a light pinch of Old Bay-style seasoning if you like.
