Combine the lentils, vegetable stock, water, onion, olive oil, tomato paste, bay leaf, salt, cumin and pepper in the slow cooker insert and stir well.
Cook on low for 6 to 7 hours, or until the soup is thick and the lentils are starting to fall apart. For a long while it will look like water with lentils in it, but suddenly it will look like soup. Wait for that.
Remove and discard the bay leaf and serve with a good splash of red wine vinegar (or two!), lots of crusty bread and lots of feta cheese. Personally, I like to crumble feta on top so that it imparts its flavor into the soup. Don’t skip it for this recipe!