Roughly chop the fennel and leek.
Heat 2 Tbsps of olive oil in a heavy-based saucepan over medium heat. Add the leek and fennel, cover and gently sauté for 10-15 minutes, stirring regularly.
Meanwhile, peel the potato and chop it into small cubes. Crush the garlic, if used.
When the fennel and leek are soft, add the potato and garlic, then stir for a couple of minutes.
Pour in 500 ml stock, add the bay leaf and simmer for about 20-25 minutes, until the veggies are tender.
Remove the bay leaf and, using a hand-held blender or liquidiser, whizz until you get a smooth, velvety soup. Add more stock if it's too thick.
Add the lemon juice or soured cream. Taste, season with salt and pepper, and add more lemon juice/sour cream, if needed. Serve with chopped chives scattered on the top.