2 carrots, peeled and chunked
3 tablespoons olive or other oil, plus addition for topping the terrine
3 medium onions, peeled and diced
1 bunch scallions, sliced
2 pounds salmon fillets, cut into large (~2 inch) cubes
1 pound cod, flounder, rockfish, or whitefish (I used Oregon petrale sole), cut into cubes
1/2 pound smoked whitefish or mackerel (this gives a subtle smoked flavor — if you prefer, you can swap out more smoked fish for some of the fresh white fish, but you'll want to reduce the salt to accommodate)
4 large eggs
4 tablespoons matzo meal
1 tablespoon coarse salt
1 teaspoon freshly ground pepper
Place the carrot chunks in a small saucepan, and cover with water. Bring to a boil, then lower the heat and simmer until the carrots are quite tender, ~7-10 minutes. Set aside.
While the carrots are simmering, pour the oil into a large saucepan or Dutch oven, and bring to a medium-high heat. Add the onions, along with a pinch of salt, and saute, stirring, until soft and translucent but not browned, ~15 minutes (lower heat as needed to keep them from coloring). When done, stir in the scallions, cook for another minute, then turn off the heat. Let cool slightly.
Drain the carrots, and place them in a food processor, along with the onions and scallions. Puree until smooth. Toss in the eggs and matzo meal, and pulse to combine. Transfer the mixture to the bowl of a stand mixer fitted with a paddle attachment.
Place all of the fish in the bowl of the food processor, and pulse until reduced to small bits, but not totally pureed (you want a bit of texture). Transfer this to the mixer bowl with the onions and carrots. Add the eggs, matzo meal, salt and pepper, and beat on medium speed for 10 minutes.
While the mixture is beating, preheat your oven to 325° Fahrenheit. Grease a large Bundt pan, and find a casserole dish that it can fit inside. Heat a kettle of water until it's not quite boiling.
When the fish mixture has beaten, pour it into your prepared Bundt pan, then place the pan in the casserole dish. Smooth the top of the mixture with a spatula, and pour a little bit of oil over the top, then cover tightly with foil.
Place the pan-in-dish in your preheated oven, then carefully pour some of the warm water (not boiling hot, lest you shatter the pan) into the casserole dish, until it comes a few inches up the side of the Bundt pan. Bake for one hour, or until the center seems solid.
When the terrine is cooked, remove from the oven, and remove the Bundt pan from its water bath. Let the terrine cool for at least 20 minutes, and up to about an hour. When cool, slide a long knife around the inner and outer edges (both!) of the pan to free the terrine, then invert onto a flat serving plate. Cover, and refrigerate for several hours, or overnight. Slice and serve with horseradish. Keeps up to five days.