Preheat the static oven to 180 degrees Celsius, 350 degrees Fahrenheit, Gas Mark 4.
Thinly slice the fennel and coat with the olive oil, salt, and pepper and then roast on the middle shelf in the oven for 25 minutes; by the time these are roasted they should be golden and tender, with a light darker color on the edges.
Five minutes before the fennel is ready, in a large soup pot warm up on a medium flame a teaspoon of olive oil; once this is warm add the onion, leek, and potato and mix well; allow to cook for about five minutes while mixing, until the edges are slightly soft.
Once the fennel is roasted, add it to the leek and potato soup base and mix well; add the vegetable stock cube (or the equivalent in powder) and the water.
Mix the whole content of the pot and bring to a boil; lower the flame to low and allow to simmer for 20 minutes.
Remove from the heat and allow to cool down enough so that it is safe to blend in batches in a self-standing blender; after the soup has been blended, should you prefer a more liquid consistency, add some more water, but remember to bring back to a boil before serving.
Adjust the salt and pepper levels and serve hot or cold – it’s good and tasty either way!