A Gefilte Fish Recipe for People Who Think They Hate Gefilte Fish

Name: A Gefilte Fish Recipe for People Who Think They Hate Gefilte Fish
Ingredients: 3 pounds (1.4kg) ground carp, pike, and/or whitefish meat, plus reserved bone cages and heads (see notes); 4 medium onions (2 pounds; 900g total), divided; 1 large celery rib, thinly sliced crosswise; 1 fennel bulb, ends trimmed, then halved and thinly sliced; A few sprigs each fresh flat-leaf parsley and dill, plus more for garnish; 2 tablespoons (30ml) vegetable or other neutral oil; 3/4 cup matzo meal (3 1/2 ounces; 100g); 3 large eggs; 3 tablespoons (27g) Diamond Crystal kosher salt, plus more as needed (for table salt, use half as much by volume); 1 tablespoon (13g) sugar, plus more as needed; 1 teaspoon freshly ground white or black pepper; 4 medium carrots (3/4 pound; 340g), peeled and sliced on the bias 1/4 inch thick; Preserved horseradish, for serving