Just use a stock cube and add your vegetables, whichever ones you require and simmer until cooked. Add salt and pepper to taste.
I do mine with beef/ham now for the stock, but you could just use a stock cube if you didnt want to use meat.
I usually use swede, leeks, onions, carrots, parsley and pulses like lentils and barley, but not sure if you can have lentils and barley on refeed, you would need to check your refeed sheet for that.