Herring in Mustard Sour Cream on Rye Bread

Name: Herring in Mustard Sour Cream on Rye Bread
Ingredients: 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)