Heat oil in a large skillet over medium-low heat. Add onion and garlic; cook and stir until onion is translucent, about 5 minutes. Add ground beef; cook and stir until browned, 8 to 10 minutes.
Transfer beef mixture to a 6-quart slow cooker. Stir in kidney beans, diced tomatoes, tomato puree, water, green chile peppers, chili powder, salt, cumin, and black pepper.
Cook on Low until flavors combine, 4 to 6 hours.
I usually use a combination of beans like kidney and black or kidney and garbanzo. I usually use dried beans and cook them in my pressure cooker.
If you can’t find puree, just use tomato paste and add water to it to get a puree consistency.
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