Heat oil in a large heavy-based pan.
Add onions, carrots and thyme, then
gently fry for 15 mins. Stir in stock, bring
to a boil, cover, then simmer for 10 mins.
Add the chicken, remove half the
mixture, then purée with a stick blender.
Tip back into the pan with the rest of
the soup, peas and seasoning, then
simmer for 5 mins until hot through.
Mix the yogurt, garlic and lemon juice,
swirl into the soup in bowls, then serve.