Coat the potatoes in a bit of olive oil and bake in the oven at 190C / 170C fan for an hour.
Once the potato is cooked remove from the oven and quarter the potatoes, then remove the flesh of the potatoes leaving just a little bit for the tato skin croutons. Add the potato flesh to the soup and sprinkle the skins with a bit of cheese and put back in the oven to melt for another 10 minutes.
Making the soup
Melt the butter in a large saucepan. Add the onion, leek, courgette and thyme to the pan and simmer over medium to high heat until the leeks and onion start to caramelise. Depending on the heat this can take between 15 and 30 mins. Be careful not to burn the onions.
Once the onion has slightly browned and caramelised add the stock and simmer for a further 15 minutes.
Once the potato flesh from the baked potato is nice and soft add it to the soup mix and blend with a hand mixer until smooth. Add the milk, simmer on low for a minute or two (be careful so it does not separate). Season to taste.
To serve swirl in the yoghurt for extra creaminess, sprinkle with chives and the tato skin croutons.