01 - Preheat oven to 400°F. Scrub potatoes thoroughly, pat dry, and pierce each several times with a fork. Place directly on the oven rack and bake for 45 to 60 minutes until tender when pierced with a knife.
02 - While potatoes bake, heat butter and olive oil in a large skillet over medium heat. Add sliced onions and cook, stirring occasionally, until deep golden and caramelized, about 20 minutes. Season with a pinch of salt and black pepper.
03 - Remove potatoes from the oven and let cool briefly. Using a sharp knife, slice each potato lengthwise and gently scoop out some of the flesh, leaving a sturdy shell. In a bowl, combine scooped potato flesh with the caramelized onions and 1 cup shredded cheddar cheese. Mix until evenly blended. Season to taste.
04 - Spoon filling back into the potato shells, mounding slightly. Top each with remaining cheddar. Return stuffed potatoes to the oven and bake for 10 minutes more until the cheese is melted and bubbling.