Combine all ingredients except for olive oil and salt in a 6-quart slow cooker. Cook on high for 4 hours or low for 6-7 hours. Add salt to taste.
I prefer this soup as it is. It's dark and rich and earthy, and not very spicy. It thickens up the next day.
Some of the reviews on Eating Well have complained that they would like it zestier. Feel free to double the spices or add tomatoes. You could also add peppers (hot or bell).
I used a 6-quart slow cooker to make this. I think it should fit into a 4-quart one. To make a larger quantity of soup, a double batch might fit in a 6-quart slow cooker; making 50% more would be a safer bet.
Adapted from Eating Well