Preheat your oven to 375°F.
Slice the tops off your bell peppers and remove the seeds and membranes. Lightly brush the outside with olive oil and season with a little salt and pepper. Place them in a baking dish and set aside.
In a large skillet, heat olive oil over medium heat. Add the diced onion and cook until softened. Stir in the garlic and zucchini, cooking for another few minutes until everything begins to soften.
Add in the cherry tomatoes, corn, and black beans. Stir well, then mix in the cooked quinoa. Season with cumin, chili powder, salt, and pepper. Let everything cook together for about 5–7 minutes so the flavors can blend.
Spoon the filling evenly into each pepper, packing it gently so they are nice and full.
Cover the baking dish with foil and bake for about 25–30 minutes, until the peppers are tender.
While the peppers are baking, prepare the avocado topping. In a small bowl or blender, mash or blend the avocado with Greek yogurt, lime juice, and salt until smooth. Add a splash of water if needed to reach your desired consistency.
Once the peppers are done, remove them from the oven and let them cool slightly. Top each one with a generous spoonful of the avocado mixture and sprinkle with cilantro, pumpkin seeds, or red pepper flakes if you like.