This pretty pink terrine traditionally served during Passover is easier to make than you might think, and it tastes fresher than jarred gefilte fish.
Preheat oven to 325 degrees. Make the terrine: Coat a 5-by-9-inch loaf pan with 1 1/2 teaspoons olive oil. Heat remaining olive oil in a skillet over medium heat. Cook onion, stirring, until soft, 6 to 8 minutes.
Pulse halibut and salmon in a food processor until finely chopped (not 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, about 3 minutes. Beat in carrot, parsnip, and dill.
Transfer mixture to pan. Smooth top using an offset spatula. 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 terrine is firm in the center, about 45 minutes. Remove pan from water. Let cool for 10 minutes.
Meanwhile, make the aioli: Mash garlic and a pinch of salt using a mortar and pestle or the side of a knife until a paste forms. Transfer to a bowl. Whisk in egg yolk and lemon juice. Gradually whisk in oils.
Run a knife or an offset spatula around the edge of the pan to loosen terrine. Place a plate on top, then flip. Tap top and sides gently several times to help release terrine. Let cool completely. Soak up any released juices with a paper towel. Refrigerate, covered with plastic wrap, for at least 1 hour.
Uncover terrine. Slice, and serve with dollops of aioli and horseradish.