Line a sided baking sheet with parchment paper. Rub each mushroom with a bit of olive oil and set them on the sheet, stalk side up.
Preheat your oven to 375 degrees.
Once the spinach wilts, remove the pan from the heat and add the rest of the ingredients, up to and including the nutritional yeast. Stir to combine.
Divide the mixture in four; I spread the mixture out evenly in the pan and using my spoon, just slice through like a pie to divide equally. Stuff each mushroom with a quarter of the mixture.
Bake in the oven for about 35 to 40 minutes. When there's 5 minutes left on the timer, put the marinara sauce in a small sauce pan or pot on medium low. As soon as your sauce is hot enough, turn off heat.
Take mushrooms out of oven, and top each with about 2-3 tablespoons of the marinara and 1 tablespoon of the vegan cheese. Continue baking for another 10 to 12 minutes, or until the cheese is just beginning to get golden on top.
* Some vegan cheese may have wheat starch or gluten, so it can be omitted if necessary for a gluten free option!