Combine oil, onion, cumin, chili powder, and oregano in a microwave-safe bowl. Microwave on high for 2 minutes until the onions soften and the spices are fragrant. (This is just to get the onions and spices started cooking so they don’t go into the slow cooker raw. If you prefer, you can also saute all of these ingredients on the stovetop for 4 to 5 minutes.)
Transfer onion and spices to the bowl of a slow cooker. Add tomato paste, garlic, corn, tomatoes (including liquid), garbanzo beans, black beans, kidney beans, and stock. (Note: To freeze, combine all ingredients except 4 cups of stock in a large freezer-safe plastic bag or container.)
Cook on low for 3 to 6 hours. (Note: the flavors will be well-developed after 3 hours, but if you prefer to have the soup simmering all day, you can let it simmer for 6 hours and then hold on the warm setting for up to 2 more hours.)
When soup is done cooking, stir in lime juice. Taste and season with some salt and pepper.
Serve soup with any toppings you’d like.