Using cheesecloth or your hands, wring out the spinach to get out as much water as possible.
Cook the onions in olive oil over medium heat until translucent. Add the mushrooms, spinach, salt, pepper, and nutmeg. Cook 5-10 minutes.
Transfer the mixture to a medium/large bowl. Mix in the mozzarella, ricotta, parmesan, and egg.
Bring a pot of water to boil. In the meantime, continue with the following steps.
Lay out a grid of wonton wrappers. (I do 12 at a time on a large cutting board.)
Fill each wonton with 1 tbsp of the mixture.
Using a pastry brush, paint the perimeter of the wonton wrapper with the egg wash, cover with another wonton wrapper, and press the edges to seal trying not to trap air with the filling.
As you finish each ravioli, place on a damp towel (or paper towel) so they don't stick.
Once the water is boiling, add the wonton ravioli ~5 at a time. You don't want to crowd them in the pot or they will stick to each other. When they come to the surface they are cooked. Add them to the sauce to finish them or plate.
Buen provecho!
Notes
As you finish each ravioli, place on a damp towel (or paper towel) so they don't stick.Chop the onions and mushrooms finely so the filling isn't too chunky. It will also cook faster.You can freeze them on a baking sheet. Once they are completely frozen, transfer them to a freezer bag. They will keep in the freezer for up to 6 months.Check out this list of the best sauces for gnocchi.