Preparation

• Position a rack in the center of the oven and heat the oven to 400°F. Place the potatoes directly on the oven rack and bake until tender when pierced with a fork, about 1 hour 10 minutes. Transfer the potatoes to a cutting board and let sit until cool enough to handle, about 10 minutes.
• Cut a 1/2-inch-thick slice lengthwise off the top of each potato; scrape the flesh from the slices into a large bowl and discard the skins. Spoon the flesh from the potatoes into the bowl, leaving a 1/4-inch shell. Add the butter, 3/4 tsp. salt, and 1/2 tsp. pepper and mix with a potato masher until blended; do not overwork (lumps are OK). Gently fold in the caramelized onions, sour cream and half of the cheese with a silicone spatula. Season to taste with more salt and pepper.
• Using your fingers, rub the outsides of the potato shells with the oil. Distribute the filling among the shells, mounding it, and place on a rimmed baking sheet.
• Bake the potatoes, uncovered, until heated through, 25 to 30 minutes. (To check, insert the blade of a small knife into the center of a potato for 5 seconds; it should be hot.) Sprinkle the potatoes with the remaining cheese and bake until the cheese melts and browns lightly in some places, 2 to 3 minutes more. Let sit 5 to 10 minutes, and serve.

Make Ahead Tips

The potatoes can be baked and filled up to 1 day ahead. Cover with plastic wrap and refrigerate. Bring them to room temperature and heat the oven to 400°F before doing the final bake.

Twice-Baked Potato Halves: After the first bake, halve the potatoes lengthwise. Scoop the flesh out of both halves, leaving a 1/4-inch shell. Make the filling, then fill both halves of each potato, bake, and finish as directed.