In a large skillet over medium-high heat, heat oil. Add beef chunks and cook, stirring occasionally, until browned on all sides. Remove from heat and add to slow cooker.
In same skillet, saute onion, bell pepper, and garlic for about 2 minutes. Add chili powder, cumin, salt, and pepper and toss to combine. Empty mixture into slow cooker and add diced tomatoes, green chilies, and tomato sauce. Stir to combine and mix everything well.
Cook for 10 hours on low. 30 minutes before serving, add beans and cook until heated through.
Serve with shredded cheese, chopped green onions, and sour cream, if desired.
Recipe by Simple, Sweet & Savory at https://www.simplesweetsavory.com/slow-cooker-beef-chili/