Preheat the oven to 350°F (180°C).
Cook couscous: Boil 1 cup water (or vegetable broth) in a small saucepan. Remove pan from heat, add couscous, stir, and cover. Let sit 5 minutes. Uncover and fluff with a fork.
Meanwhile, cut peppers in half lengthwise. Remove stem and clean out inner ribs with a spoon. Set aside.
Slice green onions.
In a large bowl, add green onions, black beans (rinsed and drained), tomatoes (with juice), nutritional yeast, cumin, smoked paprika, and chili powder. Add cooked/fluffed couscous. Stir well to combine.
Generously fill each pepper half with the couscous filling. Lightly pack down with a spoon to compact and fill the crevices. (You may have a little filling leftover. It's great as a salad, or in a burrito.)
Place pepper halves face up in a 9×13" baking dish. Add about 1/3 cup water to the bottom of the pan. Then cover with tin foil. Bake for 40 minutes.
Remove foil and bake 10 more minutes. (If using shredded cheese, sprinkle it on top for the last 10 minutes of bake time.) Garnish with any toppings before serving.
