Mix together the flour, eggs, and salt, as if making pasta. You will probably need to add about 5 to 6 tablespoons of water to reach the desired elasticity in the dough. Work it on a floured board, kneading for about 10 minutes. When it's smooth and elastic, pull it into something resembling a square. Cover with plastic wrap and let sit at room temperature for about 20 minutes.
Place the chicken fat in a heavy saute pan over high heat. Add the onions and saute until the onions are medium-brown, about 10 minutes. Place the onions in the work bowl of a food processor, and add the sliced beef and allspice. Puree until smooth. Taste, and season well with salt and pepper.
Roll out the kreplach dough into a large square, about 1/4-inch thick. Cut into smaller squares, about 2-inches each. You should have about 24 squares. Divide the beef mixture among them, placing a tablespoon or so of the beef mixture on the center of each square. Triangular kreplach are traditional; fold each square once to form a triangle, then pinch the edges with your fingers. You could also make square or rectangular kreplach, depending on how you fold and pinch.
To cook the kreplach, drop them in a pot of boiling chicken soup. Traditionally, they are cooked for half an hour or so, until the noodle is soft. An alternative, giving the noodle a more AItalian@ bite, is to cook them for 15 minutes. Serve the kreplach in soup, 3 to 4 to each bowl.
Note: Though it is traditional to serve these kreplach in chicken soup (3 to 4 per bowl), they can also make a terrific Jewish Apasta@ dish. For authenticity's sake, you can't use dairy products in the sauce but a thickened saute of mushrooms (in a vegetable oil, of course) would be a great topping