Put the onions on a roasting tin and drizzle with a little olive oil.
Take the white papery outer layer off the garlic, but leave the bulb together. Put on a piece of foil and add some water. Close the foil up around the bulb and put on the roasting tin with the onion. Bake the onions and garlic for an hour at 200C/400F
Add the cooked onions to a large pot and then squeeze the roasted garlic out of the cloves and add to the onions. Add in the rest of the ingredients (expect the yoghurt) and a little salt and pepper and heat for 15 minutes.
Blend the soup and pour into bowls. Add some Greek yoghurt and sprinkle over a little extra parsley.