Add the quinoa and water to a pot.
Bring the quinoa to a boil on the stovetop.
Reduce the heat to low and simmer the quinoa, covered, until it is fluffy and has absorbed all the liquid (it will take about 15 minutes).
Preheat the oven to 350° F.
In a large mixing bowl, combine the cooked quinoa, yellow onion, celery, garlic, nutritional yeast seasoning, salt, pepper, and dried parsley.
Place the portobello mushrooms face up on a parchment lined baking sheet.
Stuff the mushrooms with the quinoa filling mixture.
Bake the mushrooms in the oven until soft, about 16 - 18 minutes.
Sprinkle the top of each stuffed mushroom with the vegan cheese shreds.
Bake the mushrooms for a few more minutes to allow the cheese to melt.
Dig in and enjoy your stuffed mushrooms!