Add the beef, canned tomatoes with juice, and chopped onions to a 3.5 L crockpot. Mix well. Cover and simmer on low heat for 9-11 hours until the meat is tender.
Add the black beans with chili sauce, taco spice mix, and drained corn to the crockpot. Stir well. Simmer on high heat for 15-30 minutes.
Serve the stew in bowls. Top each with sour cream and grated cheese. If using the optional pepper, garnish with sliced pepper.