Preheat oven to 400°F. Prick potatoes all over with a fork then arrange on a baking sheet and bake until tender, 45 to 50 minutes. Set aside until cool enough to handle, then halve lengthwise and scoop out flesh, leaving only about 1/4 inch of potato all around. (Save potato flesh for another use.)
Meanwhile, heat oil in a large skillet over medium high heat. Add onions, salt and pepper and cook, stirring often, until golden brown, about 10 minutes. Add lime juice and cumin (if using) and stir to scrape up any browned bits. Reduce heat to medium and cook until deep golden brown, 6 to 8 minutes more. Combine sour cream, salsa, salt and pepper in a bowl; set aside.
Preheat broiler. Divide half of the cheese among potato skins, then top with onions. Arrange chicken on onions then top with remaining cheese. Broil until golden brown and bubbly, 2 to 3 minutes. Transfer potato skins to a platter and serve with salsa sour cream on the side.
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