Heat oven to 375°F. Grease a 9×13-inch baking dish.
Boil macaroni in well-salted water until just shy of al dente. Drain and set aside.
Stir topping: combine melted butter with panko and 1/4 cup Parmesan. Set aside.
Make the roux: melt 3 tbsp butter in a large pot over medium heat. Whisk in flour for 2 minutes.
Slowly whisk in milk and cream until smooth. Simmer, whisking, until thickened, 4 to 6 minutes.
Season the sauce with Dijon, garlic, onion powder, paprika, cayenne, salt, and pepper.
Take off heat. Stir in cheddar, Gruyère, and 1/2 cup Parmesan until melted and silky.
Fold in lemon zest and lobster, then gently mix in the pasta. Taste and adjust seasoning.
Spread into the baking dish. Top evenly with the buttered panko and any remaining Parmesan.
Bake 18 to 22 minutes, until bubbling and golden. Rest 5 minutes, then garnish with parsley.