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.
nutrition information (per serving):
32, Fat Calories
280, Saturated Fat
14, Monounsaturated Fat
63, Polyunsaturated Fat
Photo: Scott Phillips