Melt the butter in a large sauté pan with a lid. Fry the onion gently for about 5 mins, until softened. Meanwhile, pull any damaged or coarse outer leaves from the lettuces and trim the bases. Cut the lettuces in half lengthways, through the base.
Place the lettuce cut-side up on top of the onions and cook for half a minute, then turn over and cook for another 30 secs. Pour over the stock and lightly season. Put the lid on the pan, reduce the heat to the minimum and cook for 10 mins.
With a slotted spoon, lift the lettuces out and put into a sieve over a bowl. Raise the heat under the pan and boil the juices until reduced by half. Add the cream or crème fraîche and petits pois and boil for 1 min until heated through. Arrange lettuces in a serving dish and pour over the pea-and-cream sauce.