¼ cup rendered chicken fat (schmaltz) or vegetable oil, plus additional as needed to taste
2 onions, thinly sliced
½ pound chicken livers, trimmed of any membrane and dried with paper towel (optional)
1 pound leftover brisket, roughly cut
2 cooked carrots
2 cooked parsnips
salt and pepper to taste
In a large skillet, heat the fat over a medium-high flame. Add the onions, along with a sprinkling of salt, and cook, stirring occasionally to break them up, until the onions soften and become translucent. Turn the heat down to low, and continue to cook until the onions get an even, deep brown color, ~40 minutes. Stir only as needed. Set aside to cool slightly.
While the onions are cooking, heat a skillet to a high heat, and add a splash of fat. Add the livers (beware of spitting oil), and cook until they brown, then turn and brown on the other side. You want to get some color, but don't overcook — you want to just feel the beginnings of firmness. Set aside to cool slightly.
In a food processor, pulse the leftover brisket until it's in small shreds but still has some texture. Remove and set aside. Add the carrots and parsnips, onions, and liver, and pulse until somewhat smooth, with a bit of texture (you can also chop everything in a grinder, if you have one). Remove the mixture, and combine with the brisket. Season to taste with salt and black pepper, adding additional schmaltz or broth to moisten the mixture if needed.
2 cups unbleached all-purpose flour, plus more for dusting
½ teaspoon kosher salt
3 large eggs, beaten
Combine the flour and salt in the work bowl of a food processor. Pulse to distribute the salt. Add the eggs and 1 tablespoon water and process until a ball forms, 15 to 30 seconds. If after 15 seconds the dough doesn't begin to form a ball (it will probably appear sandy or pebbly), add a little more water (just a teaspoon at a time) without stopping the machine until a ball forms. Turn the dough out onto a lightly floured work surface and knead until the dough is smooth and supple, about 2 minutes. Form the dough into a ball, and then use a knife to divide it into 4 equal pieces. Shape each piece into a fat disk. Wrap each disk in plastic wrap and set aside at room temperature to rest for at least 20 minutes, and up to 3 hours, before using.
To shape the kreplach
Roll out the first ball of dough, either by hand or using a pasta machine. Roll until you can almost read through it (usually the next-smallest setting on a pasta machine, or until your arm nearly falls off by hand), using additional flour for dusting as needed. If the dough resists, let it relax for a minute, then come back to it.
When the dough is thin enough, use a sharp knife to cut it into 2-inch squares. Place a small amount of filling in the center of each square (~2 teaspoons), and fold the dough over it. Moisten the edges of the dough with water or beaten egg, and fold over, crimping around the filling to squeeze out the air. Press the edges to seal (with either your fingers or a fork). You can bring the two small angles together, for a tortellini shape, or leave as a triangle. Place the shaped kreplach on a flour-dusted tray, and repeat with a remaining dough and filling.
To cook the kreplach
Bring a large bowl of salted water to a boil. Gently plop in the kreplach, and adjust the heat so that it is simmering but not violently boiling. Cook the kreplach until the pasta dough is tender, a few minutes after they've risen to the top. Remove with a spider or slotted spoon, toss with a bit of oil or schmaltz, and spread out on a sheet pan so that they don't stick. Serve in hot soup, or freeze for future meals.