1 pound shaped pasta, such as elbows or shells
8 Tbs. salted butter, divided
1 small yellow onion, diced
1 garlic clove, minced
5 Tbs. all-purpose flour
2 cups light cream
2 cups shredded Gruyere cheese
3 cups shredded cheddar cheese
1/2 cup grated Romano cheese
Kosher or sea salt and freshly ground black pepper to taste
8 ounces cooked lobster meat (from about two 1-1/2-pound lobsters), cut into bite-size pieces
For the topping:
2 Tbs. salted butter
3/4 cup panko breadcrumbs
1 tsp. chopped fresh parsley
Cook pasta according to package instructions. Drain in a colander, set into the sink, and rinse with cold water to cool. Set aside.
Preheat oven to 350°. Melt 3 tablespoons butter in a medium-size saucepan over medium heat. Stir in onion and garlic; cook until onion has softened and turned translucent, about 5 minutes. Transfer to a bowl and set aside.
Melt remaining 5 tablespoons butter in the saucepan. Whisk in flour and stir until mixture is light golden-brown and glossy, 8 to 10 minutes. Gradually whisk cream into flour mixture, increase heat to medium-high, and bring to a simmer. Once simmering, reduce heat to medium-low; cook, stirring, until mixture is thick, creamy, and smooth, 10 to 15 minutes.
Stir onion/garlic mixture and cheeses into cream mixture until melted and smooth. Season to taste with salt and pepper; then gently stir in lobster and reserved pasta. Pour mixture into a 4-quart casserole and smooth the top.
Next, make the topping: In a small saucepan over medium heat, melt 2 tablespoons butter. Add breadcrumbs and parsley; cook, stirring , until lightly toasted. Sprinkle evenly over casserole.
Bake in preheated oven until sauce is bubbly and the top is golden-brown, 10 to 12 minutes.