For the stock:
2 tablespoons extra virgin olive oil
1 onion, coarsely chopped
3 stalks celery, coarsely chopped
1 leek, white part only, cleaned and coarsely chopped
1/2 bulb fennel, coarsely chopped
1 to 2 pounds fish bones
6 to 8 cups water (or enough to cover bones)
1 teaspoon kosher salt
1/2 teaspoon dried thyme
1 bay leaf
2 to 4 sprigs flat-leaf parsley
For the gefilte fish:
1-1/2 pounds freshwater or salt water white- fleshed fish fillets such as pike, carp or sea bass
1 onion, finely chopped
1 teaspoon kosher salt
1/2 teaspoon pepper
1/4 cup matzoh meal (salt-free and with no MSG)
1 carrot, peeled and thinly sliced
Horseradish to taste
To make the stock: In a medium sauté pan or skillet, heat the olive oil over medium heat and add the onion, celery, leek and fennel. Cover and cook for about 15 minutes; do not brown.
Meanwhile, cut the bones into 3- to 4-inch pieces, rinse and put in a medium pot. Add the water and bring to a boil. Skim the foam off and discard, then reduce heat to simmer. Add the salt, thyme, bay leaf, parsley and sautéed vegetables. Cover and simmer for 30 to 45 minutes. Strain through a fine-mesh sieve and reserve.
To make the gefilte fish: Grind the fish fillets in a hand grinder or food processor to a texture that resembles raw hamburger. Place the fish in a large mixing bowl. Add the onion, eggs, salt and pepper. Using your hands or a wooden spoon, thoroughly blend the fish mixture. Blend in the matzoh meal to add density. Shape the fish mix into egg-sized dumplings and reserve. (Don't make the dumplings too big; they will expand as they cook.)
In a large pot, bring the stock to a boil and reduce to simmer. Use a large spoon or ladle to gently lower the dumplings into the stock. Add the sliced carrot and simmer over medium heat until the dumplings are firm, about 1 hour.
Place the gefilte fish in a casserole or high-sided serving plate. Cover with the stock and carrots and let cool to room temperature. Refrigerate for several hours or overnight. Depending upon the natural fat content of the fish used in the stock, the liquid may or may not gel.
Serve 2 fish dumplings per person in a bit of their liquid or jelly, topped with a few carrots, and flanked by a dollop of white or red horseradish. Serve cold.