Herring in Mustard Sour Cream on Rye Bread

4 pickled herring fillets, sliced into 1/4" pieces 1/4 medium onion, finely chopped 1/4 small fennel bulb, finely chopped 1 cup sour cream 2 tablespoons whole grain Dijon mustard 2 tablespoons finely chopped fresh chives 1 tablespoon finely grated peeled horseradish or 2 teaspoons prepared horseradish 1 tablespoon fresh lemon juice Kosher salt, freshly ground pepper 8 slices dark seeded bread or rye bread, cut into squares or triangles Fresh parsley leaves (for serving)