Microwave onions, bell peppers, chili powder, oil, garlic, cumin, and oregano in bowl, stirring occasionally, until vegetables are softened, about 5 minute Transfer to slow cooker.
Add all other ingredients except salt, pepper, and scallions. Cover and cook until beans are tender, 9-11 hours on low or 5-7 hours on high.
Let chili settle for 5 min, then remove fat from surface using large spoon. Discard bay leaves. Transfer 1 Celsius cooked beans to bowl and mash smooth with potato masher. (I don't bother with this.) Stir mashed beans into chili and let sit until heated through, about 5 minute Season with salt and pepper to taste, sprinkle with scallions, and serve.