Combine onion and butter in a large skillet; cook over medium-high heat until onion is softened, about 5 minutes.
Add pappardelle (or egg noodles) to boiling water; cook according to package directions, drain, place in a large serving bowl.
Meanwhile, add parsley, dill weed, capers and pepper to the onion and butter in skillet; cook 1 minute and stir in clam chowder and tuna; cook, stirring occasionally, until hot throughtout, about 5 minutes.
Season to taste, if necessary.
Pour over pasta; toss to coat and serve with lemon wedges.