• Preheat oven to 400° Fahrenheit. If needed, adjust racks so baking dish will sit in center of oven. Lightly grease baking dish with cooking spray or butter, making sure to cover bottom and all sides completely. Set greased baking dish aside.

For the Pasta

• Fill large pot with enough water to cover pasta by 1 to 2 inches. Place pot over medium-high heat and warm pot until water begins to boil gently.
• Once water begins to boil, generously salt water, then continue warming pot until water begins to boil.
• When salted water begins to boil, add dry pasta to water and cook 1 to 2 minutes less than indicated on pasta packaging, so that pasta is just shy of al dente.
• Pour pasta and water into colander. Drain pasta and set aside.
• Dry pot with paper towels and return pot to stovetop. Reduce heat to medium.

For the Lobster Sauce

• Heat pot over medium heat until warm, then add butter. Melt butter completely, swirling pot occasionally to evenly distribute butter around pot.
• When butter is completely melted, add flour to pot and whisk until ingredients are fully combined. Let mixture simmer over medium heat 2 minutes or until lightly golden.
  
• Remove pot from heat. Whisking constantly, add whole milk to pot. Return pot to heat and continue whisking until mixture has thickened, approximately 5 minutes.
  
• Remove pot from heat once more. Add gruyere, cheddar, salt, pepper, and nutmeg. Whisk well to incorporate ingredients and continue whisking until cheeses have melted. Return pot to heat.
  
• Add lobster meat and cooked pasta to pot with cheese sauce. Gently fold in lobster and pasta until fully incorporated and coated in cheese sauce.
  
• Once incorporated, transfer entire mixture to greased baking dish. Spread mixture out into one even layer, covering bottom of baking dish entirely.

For the Lobster Mac and Cheese

• Place 2 tablespoons butter in microwave-safe bowl. Microwave in 15-second bursts at 50% power, stirring well between each burst, until butter is completely melted but not bubbling. Carefully remove bowl from microwave.
• Add breadcrumbs and parmesan to bowl with melted butter. Use fork or small whisk to mix ingredients together until crumbly topping forms.
  
• Sprinkle breadcrumb mixture evenly over top of mac and cheese in baking dish, then place baking dish in preheated oven.
• Bake mac and cheese, uncovered, 25 to 30 minutes. After 25 minutes, begin checking doneness - mac and cheese is ready when sauce is bubbly and breadcrumb topping is lightly golden brown.
  
• Carefully remove baking dish from oven. Let stand 3 to 5 minutes, then divide lobster mac and cheese into preferred portions and serve as desired.