Preheat oven to 375 degrees Fahrenheit. Line a large baking sheet with parchment paper.
If you have large Portobello mushrooms, you'll need to pre-bake them: place them upside down on the baking sheet and bake for 3-5 minutes, or until slightly softened (not cooked all the way through!)
Heat the olive oil in a large skillet and the diced onion and pepper over medium heat until tender, about 5-10 minutes. Add the minced garlic and sauté for a few minutes until fragrant.
Add diced zucchini and sauté until soft. If using Portobello stems (see notes), sauté them with the zucchini. Add the corn and fresh spinach and sauté for 2 minutes, or until the spinach wilts.
Remove from heat and add cooked quinoa, smoked paprika, dried oregano, and salt. Stir to combine and carefully taste to see if you need more salt.
Turn the Portobello caps right-side up on the and divide the filling between the 8 Portobello caps. Bake for 5 minutes, or until the Portobello mushrooms are releasing juice and cooked to your liking.
→ If your Portobello mushrooms still have stems on them, you can use the stems in the filling mixture! Just snap them off at the base, dice them, and add them in in step 4 with the diced zucchini.→ Mix and match your veggies: This recipe is totally flexible and customizable. You can use diced yellow squash instead of zucchini, double the spinach, skip the corn, double the quinoa, use other color bell peppers, add some frozen peas... use what you have!→ Add some cheese! You can add shredded vegan mozzarella on top of the stuffed peppers before baking, or sprinkle vegan Parmesan before serving.