1. Pour oil into a six-quart slow cooker. Add the leeks. Turn slow cooker to high and cook for 15 minutes. (You can’t really sauté in a slow cooker, but leaving it for 15 minutes encourages the leeks to start wilting and releasing some liquid.)
2. Add the celery, carrots, and zucchini, mixing after each addition. Add the grains and beans. Pour boiling water to cover all, plus a little more. (You don’t want the soup to be too thick or too thin.) Add the frozen veggies.
3. Turn the slow cooker to low and cook for 10 hours. Add the seasoning. Mix everything together. Taste and adjust seasoning if necessary.
Frozen string beans add a distinct flavor to soup. If you prefer the standard carrots/peas/corn or any other combo of mixed veggies, feel free to swap. I don’t recommend using too much frozen cauliflower or broccoli, as they will dominate the flavor.