Add milk and thyme to a saucepan and bring to the boil, turn off heat and leave to infuse for 1 hour.
In a frying pan fry the onions in butter in until soft but not coloured.
Cut the potatoes into small cubes and add to the onions.
Add the vegetable stock, bring to the boil and then reduce to a simmer until the potato is cooked.
Gently fry the courgettes until soft but not browned.
Add the courgettes to the saucepan of potatoes, onions and stock. Gently blend with stick blender.
Add the milk through a sieve and blend to a semi-smooth consistency.
Bring back to a simmer, add the cheeses and double cream and stir until all the cheese has melted.
8 servings