Scrub clams thoroughly, discarding any that are cracked or open. Place clams in a large bowl; cover with cold water, and sprinkle with cornmeal. Let stand 30 minutes. Drain and rinse clams; set aside. Discard cornmeal.
Combine clams, chopped onion, shallots, and wine in a large Dutch oven. Bring to a boil; cover, reduce heat, and simmer 6 minutes or until most clams are open. Remove clams to a serving bowl as they open; keep warm. Discard any unopened clams. Bring remaining liquid to a boil; cook over high heat until reduced to 1 cup. Pour liquid through a sieve. Pour strained liquid into a medium saucepan.
Bring liquid to a boil; reduce heat to simmer. Stir in lemon rind, lemon juice, and garlic. Remove from heat; add butter and remaining ingredients. Stir with a whisk until butter melts. Pour sauce over clams. Serve immediately.