Preheat the oven to 400 degrees.
Remove the stems from the mushrooms, and use a spoon to scrape out the dark brown insides. Then use a damp paper towel to wipe off any excess dirt on the outside.
Place the mushrooms stem side down on a lined baking sheet. Bake for 8-10 minutes, until just slightly soft.
While the mushrooms are pre-baking, prepare the filling. Start by thawing out the spinach in a bowl in the microwave, according to package instructions. Once thawed, place the spinach in a kitchen towel and squeeze out all of the excess water. This is very important, and will help the end result to not be too watered down.
Preheat a small skillet over medium heat, add the olive oil, shallots, and garlic. Saute for about 2 minutes, stirring frequently, until the shallots are starting to turn slightly brown.
Transfer the shallots to a mixing bowl, add the spinach, goat cheese, and a pinch of salt and pepper. Stir with a fork to evenly combine.
Pull the mushrooms out of the oven after they have pre-baked, let them cool for a few minutes so you can handle them. Turn them over and stuff each one with 1/4 of the spinach/goat cheese mixture. Smooth out the top and place them back in the oven.
Bake for 6-8 minutes, until the mushrooms are soft and the goat cheese is warmed all the way through. Serve and enjoy!