Cook pasta until al dente according to package directions. Drain, rinse with cold water, and drain again.
Place 4 tablespoons melted butter in a saucepot over medium-low heat. Add garlic and onion; cook 5 minutes or until onion is softened. Whisk in flour; cook 1 minute. Pour in milk; bring mixture to a boil over medium-high heat, whisking frequently. Reduce heat to medium-low, and simmer 3 minutes or until sauce is smooth and thickened.
Remove from heat, and whisk in cheeses and next 3 ingredients. Stir in 1⁄4 teaspoon salt and 1⁄4 teaspoon pepper. Fold in pasta and lobster. Pour into a greased 3-quart baking dish.
Combine crushed crackers and remaining 2 tablespoons melted butter. Stir in remaining 1⁄4 teaspoon salt and 1⁄4 teaspoon pepper. Sprinkle over casserole. Bake at 375° for 30 minutes or until crust is crisp and sauce bubbles. Let stand 5 minutes before serving.
This is truly excellent. I made it exactly as written but I did have to add another 3/4 cup of whole milk at the end for a more velvety sauce. I used 3 frozen lobster tails that I thawed and then steamed. It was the perfect amount of lobster for us but if you desire lobster in every bite I recommend using 4 or even 5 tails.
I thought this was a really good mac & cheese, probably one of the best that I have had. I just thought it was a bit too dry, so I would use more milk the next time I make it. Other then that it was great!