Prep your kitchen station: Preheat your oven to 375°F (190°C). Grab a baking dish large enough to hold all your peppers standing upright. If you don't have one, just call it "rustic style" and lay them on their sides.
Cook the quinoa: In a medium saucepan, combine the rinsed quinoa and vegetable broth. Bring to a boil, then reduce heat to low, cover, and simmer for about 15 minutes until the liquid is absorbed and quinoa is fluffy. Fluff with a fork and set aside. (Pro tip: See those little tails on the quinoa? That's how you know it's done!)
Prep the peppers: Cut the tops off the bell peppers and remove the seeds and membranes. If they wobble too much, slice a tiny bit off the bottom to create a flat surface – just don't cut all the way through, unless you enjoy cleaning quinoa off the bottom of your oven.
Pre-cook the peppers: Place the hollowed peppers in your baking dish, add about 1/4 inch of water to the bottom of the dish, cover with foil, and bake for about 15 minutes. This pre-cooking step ensures your peppers won't be raw while your filling is perfect. Nobody likes that disappointing crunch when you're expecting tender.
Make the filling: While the peppers are pre-cooking, heat olive oil in a large skillet over medium heat. Add the onion and sauté until translucent, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. (Burn the garlic and you'll be starting over – no pressure!)
Build the flavor: Add the zucchini and cook for another 3-4 minutes until it begins to soften. Stir in the drained black beans, diced tomatoes, corn, and all the spices. Let this mixture simmer for about 5 minutes, stirring occasionally.
Complete the filling: Remove the skillet from heat and stir in the cooked quinoa, fresh cilantro, and 1 cup of shredded cheese. Season with salt and pepper to taste. The filling should be moist but not soupy – if it seems too wet, let it sit for a few minutes to absorb the excess liquid.
Stuff those peppers: Remove the pre-cooked peppers from the oven and carefully pour out any water from the baking dish. Fill each pepper with the quinoa mixture, packing it in gently but not squishing it down too much. Top with additional cheese and crushed tortilla chips or breadcrumbs if using.
Bake to perfection: Return the stuffed peppers to the oven, uncovered, and bake for 15-20 minutes until the peppers are tender and the cheese is melted and bubbly. For a golden top, broil for the last 2-3 minutes – but watch closely! The difference between "beautifully browned" and "call the fire department" is about 30 seconds.
Serve and enjoy: Let the peppers cool for about 5 minutes before serving. Garnish with additional fresh cilantro, sliced avocado, and a dollop of sour cream or Greek yogurt if desired. Prepare for the "mmms" and recipe requests.