Preheat oven to 325 degrees.

Spray a 5-by-9-inch loaf pan with cooking spray.  Heat olive oil in a skillet over medium heat.   To the pan add the onion and cook until soft, 6 to 8 minutes.

In a food processor, pulse the halibut and salmon until chopped, don't let it get too smooth.

Beat eggs with a mixer on medium speed until frothy, about 2 minutes. Beat in onion, fish mixture, cold water, matzo meal, sugar, lemon juice, 2 teaspoons salt, and some pepper until well combined.  Add the carrots, parsnips, and dill, mix until combined.

Transfer mixture to prepared pan. Cover with parchment-lined foil, and transfer to a large roasting pan. Pour enough boiling water into roasting pan to come halfway up the sides of the loaf pan. Bake until the center is firm, about 45 minutes. Remove pan from water. Let cool for 10 minutes.

Can be made up to three days in advance.  Slice and serve with aiol and/or horseradish.