Preparation

• Heat the oil and butter in a cast-iron or other heavy-based 10- to 12-inch skillet. Add the onions and sugar, season with salt and pepper, and sauté over medium-high heat, stirring frequently, until lightly browned. Lower the heat and continue to cook the onions until very soft and evenly browned, stirring frequently, another 30 to 40 minutes; you may need to add a little more oil. When the onions are done, transfer them to a strainer and drain.
• Heat the oven to 375°F. If using a tart pan with a removable bottom, put it on a baking sheet. In a bowl, whisk the eggs and cream. Season with 3/4 tsp. salt, a scant 1/2 tsp. pepper, and the nutmeg. Add the drained onions and half of the cheese and blend thoroughly. Fill the prepared tart shell with the onions and custard. Top with the remaining cheese and bake until the tip of a knife comes out clean and the top of the tart is puffed and brown, 40 to 45 minutes. Let cool for at least 15 to 20 minutes before serving.