Place shallot, onion, garlic, wine & butter in a large heavy pot. Add mussels & clams, cover, and bring to a boil.
Cook, gently shaking pot once or twice, until the shells open, 3-5 minutes (discard any shells that don't open).
Transfer mussels & clams with the broth to a wide, shallow bowl. Squeeze the lemon over the mussels & clams and sprinkle with parsley. Serve immediately with giant chunks of grilled bread for dipping.
