In a medium bowl, beat together the 3 eggs with the water and salt.
In the bowl of a large food processor fitted with a dough blade, add flour. Add beaten egg mixture and process until a smooth dough forms. If too sticky, add a little flour. If too dry, add a little water.
In a large bowl, combine spinach, pork, and grated onion.
In a medium bowl, soak the 4 bread slices in milk or water for about 3 minutes. Squeeze out the liquid and crumble into the filling mixture. Stir in the 4 eggs until well blended.
Use a pasta machine or rolling pin to roll the pasta dough into thin sheets. Cut the sheets into 3 inch squares. Place a rounded tablespoon of filling into the center of each square. Use your finger to slightly moisten the edges with the egg yolk beaten in 1 teaspoon water. Fold the square to enclose the filling, pressing firmly to seal. Continue until all filling or pasta has been used.
At this point, the maultaschen can be frozen in a single layer on a cookie sheet.
To cook, boil maultaschen, a few at a time to not overcrowd, for 10 minutes, until heated through. Serve in broth sprinkled with fresh herbs and toasted breadcrumbs.