Make sure to prep your veg first - clean, trim and chop them all as you will need to move fast.
First chop all of you veggies.
Melt the butter and gently fry the shallots, garlic and leeks. Add the rest of the veg and mix well, ensuring the butter coats them nicely. Season with a little salt, cover and sweat for 10 min.
Add stock, cover and simmer for 30 min or until all vegetables are soft.
Remove from the heat and wait until the soup cools down a little before blending it. Once all nice and creamy, add yogurt and season to taste.
Serve with fried bacon/pancetta (use smoked as it tastes far nicer), chopped chives and crusty bread.
You can substitute butter with olive oil or rapeseed oil but I think butter works particularly well here.