Bring clams and 3 cups water to boil in large, covered soup kettle. Steam until clams just open, 3 to 5 minutes. Transfer clams to large bowl; cool slightly. Remove clams from shells by opening clams with a paring knife while holding over a bowl to catch juices. Next, sever the muscle from under the clam and remove it from the shell. Reserve meat in bowl and discarding shells. Mince clams; set aside. Pour clam broth into 2-quart Pyrex measuring cup, holding back last few tablespoons broth in case of sediment; set clam broth aside. (Should have about 5 cups.) Rinse and dry kettle; return to burner. Fry bacon in kettle over medium-low heat until fat renders and bacon crisps, 5 to 7 minutes. Add onion to bacon; sauté until softened, about 5 minutes. Add flour; stir until lightly colored, about 1 minute. Gradually whisk in reserved clam juice. Add potatoes, bay leaf, and thyme; simmer until potatoes are tender, about 10 minutes. Add clams, cream, parsley, and salt (if necessary) and ground pepper to taste; bring to simmer. Remove from heat and serve.