Peel and chop the onions and potatoes into small dice. Measure. Melt the butter in a heavy saucepan. As soon as it foams, add the onions and potatoes; stir until they are well coated with butter. Add the thyme leaves, season with salt and freshly-ground pepper, cover with a paper lid (to keep in the steam) and the saucepan lid, and sweat on a low heat for 10 minutes approx. The potatoes and onions should be soft but not coloured. Add the chicken stock, bring it to the boil and simmer until the potatoes are cooked, 5–8 minutes approx. Liquidise the soup and add a little cream or creamy milk. Taste and correct seasoning if necessary.
Serve in soup bowls or in a soup tureen garnished with a blob of whipped cream; sprinkle with thyme leaves and thyme or chive flowers.