Wash the mushrooms and remove the stems. Pat dry and place the mushrooms cavity side up on a baking sheet lined with parchment paper.
Preheat the oven to 400 degrees F.
Meanwhile, heat the oil in a skillet over medium heat.
Add the garlic, spinach, and rice. Saute until everything is heated through and the spinach has wilted. If the rice mixture appears dry, add a few drops of water. Season the rice mixture to taste with a little salt and pepper, if desired.
Remove the rice mixture from the heat. When cool enough to handle, spoon the mixture into the mushroom cavities.
Bake for 15 minutes.
Carefully remove the baking sheet from the oven and top each mushroom with a little vegan cheese.
Return the pan to the oven and bake for 10 minutes more, or until the mushrooms are heated through and the vegan cheese has melted.