Cook quinoa according to package directions with 1 1/2 cups water/broth. (I often use vegetable broth in place of water for more flavor.)
Meanwhile, halve bell peppers and remove stems, seeds and ribs.
Rinse and drain black beans, thaw corn, and slice green onions.
In a large mixing bowl, add cooked quinoa and all other ingredients except bell peppers. Stir to combine, and adjust taste if necessary (salt, more seasonings, nutritional yeast).
Preheat oven to 350.
In a 9×13″ baking dish, place pepper halves, and generously stuff them with the quinoa filling. Lightly press down to compact and fill all the crevices.
Cover with tin foil and bake for 35-40 minutes. Remove foil, and bake 10 minutes more.
Serve with any desired toppings.