In a large mixing bowl, combine the flour, 1/4 teaspoon of salt, and make a well in the center. In a small bowl, whisk 1 egg with 1/4 cup of water. Pour the egg mixture into the well of the dry ingredients and mix with a spoon until smooth.
Turn out dough onto a floured board and knead until soft and silky, about 6 minutes. If the dough is too wet, dust with flour and knead it in. Divide the dough into 2 even balls. Let the dough rest for 1 to 2 hours.
Meanwhile: Cook the chuck in water to cover over medium heat until tender, about 45 minutes. Drain the meat; set aside to cool to room temperature.
Heat oil in a heavy skillet over medium heat. Add the onion and beef liver and saute until the onions are soft and the liver is cooked through, about 3 to 4 minutes each side. Drain off any excess oil from the liver and onions; set aside to cool to room temperature.
With a meat grinder or in a food processor, grind the liver and the chuck. Add the remaining egg, season with salt and pepper, and process until well mixed.
Roll the dough out, 1 ball at a time, to 1/4 inch thick. Cut the dough into 3 to 3 1/2 inch squares. Arranging each square of dough with 1 corner pointing at you. Take 1 tablespoon of the meat mix and put it on the top half of the dough. Moisten 2 sides of the dough with cool water and fold the bottom half up over the top, forming a triangle. Fold the 2 opposite edges together, pinching the dough so that the kreplach resembles a tortellini. Repeat with the remaining dough squares and filling.
Bring the chicken broth to a boil in a large pot. Slide the kreplach into the boiling chicken broth and cook them through, about 5 minutes. Serve immediately.