Combine 24 ounces dried prunes, 12 ounces dried apricots, 1/2 lemon, 1 orange (cut into 8 pieces) and water to just cover in a saucepan. Bring to a boil, lower heat and simmer until water is almost gone and fruits are soft. Let cool. Grind with 1–1/2 to 2 cups walnuts. Add 1/4 to 1/2 cup sugar to taste.
An alternate approach I’ve used is to simmer 20 ounces of prunes with 1/4 to 1/2 cup of water. I cooked them for 8 minutes in the microwave until they were fully rehydrated. I ground them in the meat grinder, added the grated peel from one large orange and 1/4 cup almond meal. I added no sugar.
Extra filling can be frozen and will easily keep for a year in the freezer. If you’re not making a big batch of Hamentaschen or Fluden, you might want to cut the quantities back. See the recipes for Hamentaschen Sugar Dough and Hamentaschen Honey Dough.