Add a drizzle of vegetable oil to a heavy-bottomed pot and heat over medium-high heat. Once the oil is hot, add the sirloin and onions, and brown for about 10 minutes. Once browned, add all of the remaining ingredients to the pot and bring up to boil. Turn it down to a simmer and cook uncovered for 2 hours. Garnish individual chili bowls with cheese and sour cream. Serve with cornbread alongside.