* Cut off the dark green section of the leek and discard. Slice the white and light green section of the leek in half lengthwise and clean well, between each layer, under cold running water. Dry. Cut the leek into 1 ½ inch long pieces.
* In a large skillet heat the oil over low heat. Add the onions and cook 6 minutes, stirring occasionally. Add the leeks, butter, half the thyme, salt, and pepper, and cook another 5 minutes, stirring occasionally. Remove from the heat.
* Transfer the onion mixture to a large gratin dish or ovenproof skillet. Sprinkle the cheese on top. Pour the cream over the dish and sprinkle with the remaining thyme, salt, and pepper. The gratin can be covered and refrigerated for several hours up to this point.
* Preheat the oven to 350 degrees. Bake on the middle shelf for 20 to 30 minutes, or until bubbling and the cheese is melted. Place under the broiler for about two minutes to give the dish a darker, golden brown color, if you like.
Leeks have a subtle, sweet onion flavor that marries well with the tanginess of goat cheese. An impressive side dish that pairs deliciously with meat or poultry, this simple gratin can be made several hours ahead of time and finished off in the oven for approximately 30 minutes before serving