Gefilte Fish Terrine

Name: Gefilte Fish Terrine
Ingredients: 3 tbsp. olive oil; 1 medium white onion, minced; 12 oz. boneless, skinless carp or pike fillet, roughly chopped; 12 oz. boneless, skinless salmon fillet, roughly chopped; 1 cup heavy cream; 2 tbsp. matzo meal; 1 tbsp. minced dill, plus sprigs for garnish; 2 tsp. kosher salt, plus more to taste; 1 tsp. freshly ground pepper, plus more to taste; 2 eggs, beaten; 1 carrot, coarsely shredded; Zest of 2 lemons, plus 3 tbsp. juice, and wedges for serving; 2 cups baby arugula; 4 red radishes, thinly sliced; 1 (1) piece horseradish, peeled and finely shredded; 1 shallot, thinly sliced; Matzos, for serving