1 tablespoon vegetable or olive oil, plus more for drizzling
1 medium yellow onion, small dice
1 cup quinoa, rinsed
2 tablespoons tomato paste
2 cloves garlic, finely chopped
1 1/2 teaspoons chili powder
1 1/2 teaspoons ground cumin
2 cups water, plus more for the baking dish
1 teaspoon kosher salt, plus more as needed
Freshly ground black pepper
8 medium red, yellow, or orange bell peppers (about 4 pounds)
1 (15-ounce) can black beans, drained and rinsed
1 1/2 cups fresh or frozen corn kernels
1 1/2 cups shredded Monterey Jack or sharp cheddar cheese, or blend (optional)
1/2 cup coarsely chopped fresh cilantro
Arrange a rack in the middle of the oven and heat to 375°F. Pour just enough water (about 1 cup) to cover the bottom of a 9x13-inch baking dish; set aside.
Heat 1 tablespoon of the oil in a large saucepan over medium heat until shimmering. Add the onion, season with salt, and cook, stirring occasionally, until softened, about 8 minutes.
Add the quinoa and cook, stirring occasionally, until toasty-smelling, about 2 minutes. Add the tomato paste, garlic, chili powder, and cumin, and cook, stirring occasionally, until fragrant, about 2 minutes.
Add the 2 cups water and 1 teaspoon salt, season with pepper, and stir to combine. Bring to a boil. Reduce the heat to the lowest setting, cover with a tight-fitting lid, and cook undisturbed for 15 minutes. Meanwhile, prep the bell peppers.
Use a small knife to cut a wide circle around each bell pepper stem (like when carving a jack-o'-lantern), so you end up with a cap that can be replaced once you've stuffed the peppers. Remove and discard any seeds and membranes from both the cap and interior. Drizzle the inside of each pepper with about 1/2 teaspoon of oil and season the inside generously with salt and pepper; set aside.
When the quinoa is ready, remove from the heat and let stand covered for 5 minutes. Fluff the quinoa gently with a fork. (If any liquid remains in the bottom of the pan, drain the mixture. If the quinoa is still a bit crunchy, return the pot to low heat and cook, covered, for another 5 minutes.) Add the beans, corn, cheese (if using), and cilantro, and stir to combine. Taste and season with more salt and pepper as needed, making sure the filling is well-seasoned.
Evenly divide the mixture among the peppers. Replace the caps. Place the stuffed peppers into the prepared baking dish, wedging them together as needed to fit in one layer. Cover tightly with aluminum foil.
Bake until the peppers are softened but not mushy and the filling is heated through, about 1 hour. Uncover and let sit 5 minutes before serving.