Clam and Mussel Conserva

Melt butter in a large pot over medium-high. Add clams and wine. Cover and bring to a simmer over medium-high; cook, shaking pot occasionally, until clams open, 5 to 10 minutes. Using a slotted spoon, transfer clams to a large bowl; discard any clams that do not open. Add mussels to pot; cover and cook until mussels open, 3 to 6 minutes. Transfer mussels to bowl with clams; discard any mussels that do not open. Pour cooking liquid into a large heatproof measuring cup, and set aside. Let shellfish stand until cool enough to handle, about 15 minutes. Shuck meat, and place in a large heatproof bowl; discard shells.

Peel 3 (2-inch) strips of lemon peel from lemon using a vegetable peeler. Halve lemon; squeeze to yield 3 tablespoons juice. Set lemon peel and juice aside. Heat oil in a 12-inch skillet over medium. Add celery, carrot, shallot, and garlic; cook, stirring often, until vegetables are tender, about 6 minutes. Stir in thyme sprigs, bay leaves, and lemon peel. Carefully pour shellfish cooking liquid through a fine wire-mesh strainer into skillet, leaving any sediment in measuring cup. Bring to a gentle simmer over medium. Remove from heat; stir in lemon juice and vinegar.

Pour mixture in skillet over shellfish. Let cool 1 hour. Stir in parsley; season with salt and pepper. Cover and chill at least 6 hours. Bring to room temperature; serve with lettuce and bread.