Melt the butter in a large, heavy-based pan over a medium to low heat. Add the celeriac, leek, garlic and onion. Season generously and gently sweat the vegetables until they are all starting to soften (approximately 10 minutes).
Add the stock and bring the soup to the boil. Turn down the heat and simmer for about 30 minutes or until the celeriac is completely tender. While soup is cooking mix the yoghurt and curry paste together.
Liquidise until smooth, return to the pan and reheat over a medium flame. Just before serving, check the soup for seasoning and serve with a dollop of curried yoghurt, sprinkling the toasted sesame seeds on top.