1. Cook pasta until al dente according to package directions. Drain, rinse with cold water, and drain again.
2. 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.
3. 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.
4. 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.