Perhaps you're asking about the classic French dish Potatoes Lyonnaise, in which the onions are sautéed before being added to the potatoes. There are a number of such recipes available online (including here!).

Elizabeth David claims that her recipe, in which the potatoes are separately cooked in a sauté pan, then mixed with the onions just before serving is the authentic one. Wolfgang Puck's version is baked in the oven - after the onions have been sautéed.