How to Make It
Preheat oven to 400ºF. Place clams in a single layer in a very large pot, and add enough cold water to come up to the back hinge of clams (5 to 6 cups). Cover and bring to a simmer over high. Reduce heat as needed to maintain a low simmer; cook until clams open, about 5 minutes. Remove pot from heat, and uncover. Let stand until clams are cool enough to handle. Remove clams from pot, reserving broth. Discard any unopened clams. Remove clam meat from shells; finely chop meat, and set aside.
Using poultry shears, cut through shell hinges. Arrange half shells on an aluminum foil–lined rimmed baking sheet. Pour 2 cups clam broth into a liquid measuring cup, leaving any sand or residue in bottom of pot. Set clam broth aside. Reserve any remaining broth for another use, or discard.
Melt butter in a large skillet over medium. Add sausage, and cook, stirring occasionally, until sausage has plumped and released its red oil, 1 to 2 minutes. Remove sausage with a slotted spoon, and transfer to a plate lined with paper towels to drain. Add onion, celery, and bell pepper to skillet; stir to coat in fat. Cook, stirring often, until vegetables are softened, about 8 minutes. Add garlic, and cook, stirring often, 1 minute. Remove from heat.
Add chopped clam meat, sausage, breadcrumbs, thyme, and 1 cup reserved clam broth to skillet, and return to heat over medium. Fold to combine; cook just until heated through, about 1 minute. (If mixture is too dry, add remaining 1 cup broth, 1/4 cup at a time, as needed. Mixture should be moist but not wet.) Stir in parsley, salt, and black pepper.
Divide stuffing among clam shells on prepared baking sheet (about 1/4 loosely packed cup each). Bake in preheated oven until tops are sizzling and golden brown, about 20 minutes. Divide stuffies among 8 plates; garnish with parsley sprigs, and serve immediately with lemon wedges.