Combine onion, shallots, bay leaf, parsley stems, thyme, peppercorns and wine. Bring to a boil. Add scrubbed clams and steam for 4 to 5 minutes or until clams open. Remove clams, discarding any that haven't opened, separate them from shells and chop coarsely. Strain and save liquid. Heat olive oil and 1 TABLESPOON reserved clam juice in a medium-size non-stick frying pan. Add garlic and mustard and cook for 1 minute. Add parsley and cook 1 to 3 minutes more. Add 1 cup reserved clam juice and simmer for 5 minutes or until sauce is reduced by half. Add chopped clams and cook for 1 to 2 minutes. Toss clam sauce with linguine, garnish with lemon, if desired, and serve warm. Makes 4 servings.