Scrub clams; discard any clams that do not close when tapped. In Dutch oven or large heavy-bottomed saucepan, bring clams and 1 cup water to boil over high heat. Reduce heat to medium, cover and cook until clams have opened, about 10 minutes.
Discard any clams that do not open. Using slotted spoon, remove remaining clams to bowl. Set aside.
Strain cooking liquid through cheesecloth-lined fine-mesh sieve into bowl; pour into liquid measure. Add enough water to yield 3 1/2 cups. Set aside.
Remove meat from all but 8 of the clams to cutting board; coarsely chop meat. Refrigerate chopped clam meat and remaining clams until ready to use.
In Dutch oven or large heavy-bottomed saucepan, cook bacon over medium heat, stirring often, until crisp, about 10 minutes. Using slotted spoon, remove bacon to bowl; set aside.
Reserving 2 tbsp fat, drain. Return reserved fat to Dutch oven. Add butter; melt over medium heat. Cook shallots and celery, stirring occasionally, until softened, about 8 minutes. Add garlic, thyme and bay leaf; cook, stirring frequently, until fragrant, about 1 minute. Sprinkle with flour; cook, stirring frequently, for 1 minute.
Gradually whisk in reserved cooking liquid. Stir in potatoes and pepper; bring to boil. Reduce heat, cover and simmer, stirring occasionally, until potatoes are fork-tender, about 20 minutes.
Discard thyme and bay leaf. Stir in sherry, lemon juice and Worcestershire sauce; bring to simmer. Cook, stirring occasionally, for 5 minutes. Stir in cream, chopped clam meat and bacon; cook until heated through, about 3 minutes. Stir in chives and parsley.
Ladle chowder into serving bowls; top with reserved clams.
Tip from The Test Kitchen: If any clams remain closed after the 10 minutes of cooking, remove the open ones to a bowl. Continue to cook the remaining clams, covered, for an additional two minutes, then give them a stir. If they still don't open, discard them.
A hollowed-out roll with a thick, sturdy crust makes a great edible bowl!