In a large skillet, warm oil over medium-high heat. Add onion and chili powder. Cook, stirring often, until softened, 3 to 5 minutes. Add beef, salt and pepper and cook, stirring, until crumbly and no longer pink, about 5 minutes. Transfer contents of skillet to a slow cooker.
Add tomatoes and beans. Stir well, cover and cook on low for 8 hours. Top chili with condiments such as shredded cheese, sliced jalapeños, diced onions and sour cream.