1. If there are any bones left in your fillets, remove the larger ones by hand, but don’t fret about the smaller ones since they’ll be pulverized in the food processor. You can buy your fish preground from a fishmonger (usually a Jewish fishmonger) to ensure all the bones are removed, but try to cook your fish that day since ground fish loses its freshness faster.
2. Place the onion in the bowl of a large food processor and process until finely ground and mostly liquefied. Add the fish fillets to the food processor along with the rest of the ingredients, except for the horseradish. Pulse in the food processor until the mixture is light-colored and evenly textured throughout. Scoop into a bowl and give it an additional stir to ensure that all the ingredients are evenly distributed throughout.
3. Preheat the oven to 350oF. Line an 8 x 3-inch loaf pan with parchment paper and fill the pan with the fish mixture. Smooth out with a spatula.
4. Place the loaf pan on a baking sheet on the middle rack of the oven and bake for 40 to 45 minutes. The terrine is finished when the corners and ends begin to brown. The loaf will give off some liquid. Cool to room temperature before removing from the pan and slicing.
Serve with horseradish relish.