Peel and finely chop the onion. Trim the very tops and bottoms off the leeks. Halve it and rinse out the grit. Finely slice them. Warm a large pan for 1 min. Add 1 tbsp olive oil and the onion and leek. Season with salt and pepper. Cover, keep the heat low, and sweat for 5 mins.
Peel and finely chop the potato. Add it to the pan with 1 bay leaf. Crumble in the stock cube. Pour in 800ml boiling water. Cover, turn up the heat and bring to the boil. Simmer for 20 mins till the potatoes are tender.
Fish out the bay leaf. Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Taste and add more salt and pepper if you think it needs it. Serve in warm bowls with freshly ground pepper.