Lobster Thermidor

Name: Lobster Thermidor
Ingredients: 2 lemons, halved; 1 onion, quartered; 1 Bouquet Garni, recipe follows*; 2 (1 1/2 to 1 3/4-pound) live Maine lobsters; 2 tablespoons unsalted butter; 2 tablespoons minced shallots; 1/2 teaspoon minced garlic; 2 tablespoons all-purpose flour; 2 tablespoons cognac or brandy; 3/4 cup milk; 1/4 cup heavy cream; 1/4 teaspoon salt, plus 1/8 teaspoon; 1/8 teaspoon ground white pepper; 1/2 cup finely grated Parmesan, plus 2 tablespoons; 1 tablespoon dry mustard powder; 1 tablespoon finely chopped fresh tarragon leaves; 2 teaspoons finely chopped parsley, plus additional for garnish; 1 bay leaf; 1 small sprig fresh thyme; 3 sprigs fresh parsley; 1/2 teaspoon black peppercorns