1. Make salmon: Place fillets in a large pan in a single layer and add enough water to cover by 1 inch. Add onion, celery, carrot, peppercorns, salt, bay leaf and lemon juice. Bring to a boil over high heat, then reduce heat until water is barely simmering. Simmer until salmon fillets are cooked through and beginning to flake, about 10 minutes. Gently remove salmon from water and transfer to a wire rack set over a baking sheet and let cool to room temperature.
2. Make sauce: In a medium bowl, combine sour cream, mustard, lemon juice, dill, parsley, zest, salt and pepper and mix well to combine.
3. Arrange salmon on a platter surrounded by lemon wedges and sprigs of parsley and dill. Serve with mustard-herb sauce on side.