In a small bowl, whisk together the egg yolk and water.
Divide the dough into four equal pieces.
Place one piece on a floured work surface and cover the remainder. Line a large baking sheet with parchment.
Roll the dough into a thin sheet using a pasta machine or rolling pin, until it is about 1/16th inch (1.5 millimeter) thick.
To form individual squares: Cut the sheet of pasta into 3-4 inch (7.5-10 centimeter) squares.
Place a spoonful of the pork and spinach filling in the center of the square, leaving about 1/4-1/2 inch (6-12.5 millimeters) free around the edges.
Brush the edges with the beaten egg wash and fold the square of pasta over, sealing the edges.
If desired, press a fork around the edges to seal well. Place on the parchment-lined baking sheet and repeat with remaining dough and filling.
To form strudel-like Maultaschen: After rolling the dough into a thin sheet, spread the filling lengthwise across the sheet, leaving about 1-2 inches (2.5-5 centimeters) free on each side.
Brush the free edges with the egg wash and roll up the dough tightly, long side to long side, pressing out any air.
Use a wooden handle to press down on the roll of dough and create dumplings, about 2 inches (5 centimeters) apart. Cut along the separated, pressed down edges to form the individual Maultaschen and arrange in a single layer on the parchment-lined baking sheet.
Repeat with remaining dough and filling.
Bring a large pot of salted water to a boil. Reduce heat to a light simmer and cook the Maultaschen, a few at a time to not overcrowd, until floating on the top and cooked through, about 10 minutes.
Serve in broth with fresh herbs or slice and pan-fry in butter with caramelized onions.