Preheat oven to 375 degrees F.
In a skillet, over medium heat, sauté onions and mushrooms stems with olive oil for 8-10 minutes or until onions are translucent.
Add roasted red peppers and spinach and heat another 2-3 minutes. Remove from heat and stir in nutritional yeast. Add sea salt and pepper to taste.
Dividing evenly, fill mushroom caps with filling. Bake for 25-30 minutes.
Drizzle with balsamic reduction before serving if desired.