Preheat oven to 400°F (200°C). Line a rimmed baking sheet with parchment or foil. This temperature roasts the peppers until tender without drying them out and will finish the stuffed peppers quickly in the oven.

Prepare the peppers: Slice each large bell pepper in half lengthwise and remove seeds and membranes. Lightly brush the pepper interiors with 1 tablespoon oil and season with a pinch of salt. Arrange cut-side down on the prepared sheet. Roasting the peppers cut-side down helps steam and soften the flesh while concentrating sweetness; it also creates a slightly blistered interior that holds the filling better. Roast for 12–15 minutes until edges are just softened but peppers still hold their shape.

Meanwhile, heat 1 tablespoon olive oil in a skillet over medium heat. Add the diced onion and a pinch of salt and cook, stirring occasionally, until translucent and beginning to brown at the edges (about 5–6 minutes). Browning develops savory compounds through the Maillard reaction and builds depth for the filling.

Add minced garlic and toast briefly (30 seconds to 1 minute) until fragrant — you want the garlic softened and aromatic but not bitter. Then stir in the ground cumin and toast for 20–30 seconds to bloom the spice and release its essential oils; this deepens the cumin flavor in the final filling.

Lower heat and add the cooked quinoa, black beans, corn and diced tomato to the skillet. Stir to combine and warm through (2–3 minutes). Warming the beans and quinoa allows the flavors to meld and ensures the filling is hot when it goes into the peppers so the oven time only finishes and melds flavors rather than cooking the filling from cold.

Season the filling with 1 teaspoon salt, 1/4 teaspoon black pepper, the zest and juice of 1 lime, and half the chopped cilantro. Taste and adjust seasoning — the lime brightens, salt balances, and cilantro adds freshness. If the mixture seems dry, add 1–2 teaspoons of olive oil or a splash of reserved bean liquid.

Remove peppers from oven and flip them so cut-side is up. Spoon the quinoa–bean mixture into each pepper half, packing gently but not compressing (over-compressing makes the filling gummy). Fill generously — you should have enough to mound slightly in each half.

Place the stuffed peppers in a baking dish or back on the sheet. Cover loosely with foil and bake at 375°F (190°C) for 12–15 minutes to heat the filling through and allow flavors to marry. Covering retains moisture so the filling stays creamy and the peppers do not dry out.

If using cheese, remove the foil, sprinkle the shredded cheese evenly over each pepper and broil on high for 2–3 minutes (watch closely) until the cheese melts and develops little golden spots. The broil step adds a finishing, restaurant-quality touch with a melty, slightly browned top.

Finish and serve: Remove from oven, squeeze the remaining lime over the peppers, scatter the remaining cilantro, and add an extra grind of black pepper. Let rest 3–5 minutes — this short rest helps the filling set slightly so the peppers slice cleanly.

To serve: place one stuffed pepper half per person on a warm plate and garnish with avocado slices, a dollop of Greek yogurt or sour cream, or pickled jalapeños if desired. These accompaniments add creaminess, acidity or heat to round out the plate.