Tip: we use potato ricers in our cookery school all the time - for a lump-free delicious mash.
Steam the potatoes with the sprigs of thyme until cooked.
Heat the butter and cream/milk with the spring onions, salt and pepper in a medium saucepan over a low heat. (Though I usually allow a little more, depending on the potato type, as some absorb more liquid than others.
Push the warm potatoes through a ricer into a bowl and then stir in the warm cream and butter mixture until soft and smooth. Stir well. Check for seasoning and add the herbs. Serve with Beef and Irish stout pie.