Recipe: Sweet potato white bean soup

This is another slow cooker recipe that I have lifted from 'The Vegan Slow Cooker' by Kathy Hester.
It calls for the use of Swiss chard which I could not find in the supermarkets. I did end up finding it in a farmer's market, like the Stirling Markets in Stirling but that was after I made this. I substituted the Swiss chard with sliver beet which worked well but next time I will give it a go with the Swiss chard, now that I know where to find it.

What you will need:
1 medium sized sweet potato, chopped
2 cloves garlic, minced (optional)
2 stalks celery, chopped
1 can diced tomatoes
1 can cannellini beans, drained and rinsed
5 cups vegetable or vegan chicken-flavoured stock. (I suggest using Massel or Marigold)
2 cups chopped Swiss chard or sliver beet is you can't find the chard. (If you prefer, stem parts can  be discarded and use onlt the green leafy parts of the silver beet.)
1 tbsp minced fresh thyme or half that if using dried
2 sprigs parsley, chopped

Method:
Combine the sweet potato, garlic, celery, tomatos, beans and stock. Cook on low in the slow cooker, for 6 to 8 hours.
20 minutes before serving, add Swiss chard or silver beet , thyme and parsley. Cook until the Swiss chard or silver beet is tender. Add salt to taste, if desired.