Preheat oven to 350 degrees F (176 C) and rinse rice in a fine mesh strainer.
Bring veggie stock to a boil in a small saucepan, then add rice. Lower heat, cover and cook until liquid is completely absorbed – anywhere from 30-45 minutes. Taste to sample doneness – if you prefer a chewier, softer rice, cook longer. If you’re OK with more bite, check around the 25-minute mark.
In the meantime, place walnuts on a baking sheet and toast in oven for 5 minutes. Set aside.
Brush dirt from mushrooms using a damp towel and remove stems. Brush or spray with olive oil and set aside.
Prepare parmesan cheese if needed. Set aside.
Once rice is done, fluff and then add vegan parmesan cheese, toasted walnuts, minced garlic and olive oil. Stir, taste and adjust seasonings as needed. I added just a pinch of salt and pepper and a little more vegan parmesan.
Bake the mushrooms on a baking sheet WITHOUT the filling for 10 minutes to soften.
Remove from oven and scoop generous spoonfuls of the filling into the par-baked mushrooms. (You will have a little leftover filling.) Top with additional vegan parmesan cheese and bake another additional 15-18 minutes or until tender and the parmesan is golden brown. (Baking the mushrooms on their own helps prevent the rice from getting too crisp.)
Serve immediately. TIP: Dust your serving plate with vegan parmesan cheese so the bottom of the mushrooms have plenty of flavor, too.
*Black rice cooking instructions adapted from The Kitchn *14 mushrooms yield ~10 ounces or 283 g *Nutrition information is a rough estimate.