Preheat oven to 400°F. Halve peppers lengthwise, keeping stems intact if present (to help maintain their shape) and remove seeds and membranes. Brush insides with oil, season with salt and pepper, and place cut-side up on a baking sheet. Roast for 15 minutes until slightly tender, then remove and set aside.
To Prepare Stuffing:
While peppers roast, prepare stuffing. In a medium pot, cook quinoa in 2 cups of low-boiling water with a pinch of salt, covered, until water is absorbed and quinoa is fluffy, about 15 minutes. Fluff with a fork and set aside.
In a large skillet, heat oil over medium-high heat, sauté onion until translucent and golden, 4-5 minutes
Add mushrooms and cook until tender and browned, 4-6 minutes.
Turn heat to medium, add the garlic and sautéfor 30-60 seconds until fragrant.
Add the tomato paste and spices, sauté for 30-60 seconds until fragrant.
Add the vegetable broth and bring to a simmer. Mix in cooked quinoa, beans, and corn, adjusting seasoning as needed, and cook uncovered for 5-8 minutes until liquid reduces, stirring often. Remove from heat.
Stuff peppers evenly with mixture, packing it down, and arrange upright on a baking sheet with space between each. Bake until quinoa is lightly browned and crispy on top, 15-20 minutes.
Notes
Topping Ideas: vegan sour cream, fresh-cut cilantro, squeeze of lime, hot sauce, salsa, avocado, and diced red onion.
If you'd like the peppers to be more tender, let them cook 10 minutes longer. Cook less if you'd like a crisper texture.
You may have filling leftover and this all depends on the size of the peppers you used. You can save it for wraps, burritos, tacos, salads, a breakfast side, or for more stuffed peppers. It will last in the fridge for 5-6 days or in the freezer for 2-3 months.