Rinse and drain mushrooms; trim and discard discolored stem ends. Thinly slice mushrooms and put in a 10- to 12-inch frying pan. Add 1/4 cup sherry, shallots, lemon juice, and 2 tablespoons butter. Stir often over high heat until juices evaporate, about 10 minutes.
Meanwhile, in a small, deep bowl, whip cream until it holds soft peaks. Flavor to taste with salt, pepper, and about 1 tablespoon sherry. If making up to 2 hours ahead, cover and chill.
In another small bowl, mix broth and cornstarch. Add to mushroom mixture and stir until boiling, about 1 minute; if making up to 2 hours ahead, remove from heat and let stand.
Butter toast, if desired, and set on dinner plates; lay lobster tail shells, cupped sides up, beside toast (if using Maine lobster, also garnish plates with body shells). Arrange a cluster of watercress sprigs on each plate.
Cut lobster tail meat into 1/2-inch chunks. Add tail and leg meat and about half the flavored whipped cream to mushroom mixture; stir over medium-high heat until boiling, 2 to 3 minutes (3 to 4 minutes if chilled). Quickly spoon mixture into tail shells and onto toast. Spoon remaining whipped cream onto lobster mixture; if available, lay claw meat on cream.
Per serving: 445 cal., 47% (207 cal.) from fat; 31 g protein; 23 g fat (13 g sat.); 28 g carbo (6 g fiber); 733 mg sodium; 146 mg chol.
How to cook and shell lobster
Maine, or American, lobsters have large, meaty claws; mostly, this shellfish is sold fresh-cooked or live for cooking. Spiny and other rock lobsters have tiny claws; all the meat is in their tails, which are widely available frozen; look for great deals at discount warehouse stores.
PREP AND COOK TIME: 10 to 20 minutes to boil water; 25 to 35 minutes to cook, cool, and shell lobsters
MAKES: A 1 1/2-pound Maine lobster yields about 4 ounces meat, a 6- to 8-ounce spiny lobster tail 4 to 6 ounces meat; each makes 1 serving
Bring 6 quarts water to a boil in an 8- to 10-quart pan over high heat. Plunge 2 live Maine lobsters (about 1 1/2 lb. each) headfirst into boiling water (or add 2 thawed frozen spiny lobster tails, 6 to 8 oz. each, to 3 qt. boiling water in a 5- to 6-qt. pan). Cover and cook 10 minutes; when boil resumes, reduce heat to a simmer. Drain lobsters and let stand until cool enough to touch.
For Maine lobsters, twist off tails (A). For both Maine lobsters and spiny lobster tails, use scissors to snip free the flat underside of each tail shell, cutting along inside edges (B); discard underside shell and flippers. Lift out meat; pull out and discard vein, if present. Put meat in a bowl. Rinse and save tail shell backs.
Twist legs with claws from Maine lobsters. Pull shells from bodies. If desired, scoop out and save red roe, green tamalley (liver), and white fat for other uses. Discard body shell interiors; rinse and save shells. If desired, reserve small legs to eat later. Break claws from large legs. Crack leg shells and remove meat; add to bowl. Insert scissor tips into open end of each claw (C); snip along ridge to tip. Break shell open; carefully remove claw. Discard shells; add meat to bowl.
If cooking up to 6 hours ahead, cover meat and seal reserved shells in a plastic bag; chill.
Per ounce: 28 cal., 6% (8 cal.) from fat; 8 g protein; 2 g fat (0 g sat.); 4 g carbo (0 g fiber); 108 mg sodium; 20 mg chol.