Lobster Macaroni and Cheese

Name: Lobster Macaroni and Cheese
Ingredients: Kosher salt, to taste; 12 oz. hollow pasta, preferably elbow macaroni; 4 tbsp. unsalted butter; 1⁄4 cup flour; 4 cups milk; 11 oz. grated fontina (about 4 cups); 8 oz. mascarpone (about 1 cup); 3 tbsp. lobster or fish broth; 3 tbsp. brandy or cognac; 1 tsp. Tabasco; 1⁄4 tsp. freshly grated nutmeg; Freshly ground black pepper, to taste; 8 oz. cooked lobster meat, cut into 1″ chunks; 1⁄3 cup minced chives; 2 scallions, thinly sliced crosswise; 2 oz. grated sharp aged white cheddar (about 1 cup)