Cut off the top and remove the seeds from each pepper. Place in a baking dish and bake for 20 minutes in a preheated oven at 350˚F (175˚C).
While the Peppers are roasting - heat oil in a large skillet over medium/high heat. Add ground beef and season with entire taco seasoning packet. Allow beef to brown on all sides. Add diced onion - continuing to cook until softened. Reduce heat to Medium. Mix in the black beans, brown rice, corn, and salsa. Add in 1 cup of cheese and stir until incorporated and the cheese has melted. Remove from heat.
Remove peppers from oven and fill each with the taco mixture. Top with additional cheese and return to the oven for 15 minutes until the cheese has melted. Top each pepper with a generous scoop of guacamole and sour cream. Enjoy!