Preheat the oven to 350ºF. Add the chorizo to a skillet and sauté over medium heat until fully cooked. Remove it from the heat.
Rinse and drain the black beans, then add them to the skillet, along with the diced tomatoes (with juices), cooked rice, chili powder, and salt. Stir to combine.
Add half of the cheese and half of the green onion to the chorizo mixture, then stir to combine.
Slice each of the bell peppers in half and then carefully remove the white ribs and seed pods, keeping the stem intact to help hold in the stuffing. Pile the chorizo and rice mixture into each pepper, making sure to stuff it up into all the crevices, and mound it up over top. Place the peppers in a casserole dish as you fill them.
Once all the peppers are filled, add about 1/2 inch of water to the dish to help the peppers steam and soften. Cover the dish tightly with foil, then bake for 30 minutes.
After 30 minutes, remove the foil and top the peppers with the remaining shredded cheese. Bake for another 10 minutes, or until the cheese is melted and bubbly on the edges. Top the peppers with the remaining sliced green onion just before serving.