Heat your oven to 400°F (204°C), and line a baking sheet with parchment.

Using a food processor, mince the onions. Remove them from the processor, and then add the mushrooms. Mince the mushrooms, and set aside.

In a large 10″ (25cm) skillet, heat the butter until it melts. Add the onions, and allow them to brown over medium heat until they are deeply caramelized, about 10 minutes. Add the minced mushrooms, and stir to combine well. The mushrooms will absorb all of the butter, but will not need to cook all that long, about 2 minutes. Stir in the minced garlic,  season with salt and pepper, and remove from the heat.

Fill each of the empanada wrappers with a tablespoon of the mushroom filling. Crimp the edges of the empanada wrapper together to seal.

Break the egg into a small bowl, and whisk to scramble. Add a tsp. of water and whisk again.

Form all of the empanadas in this fashion. Brush the tops of the empanada with the egg wash. Bake in the oven for 25 minutes, until golden brown.