Caramelize Onions:
Heat 2 tablespoons olive oil in a large cast-iron skillet over medium heat. Add onions and 1 teaspoon salt. Reduce heat to medium-low and cook, stirring occasionally, until onions are deep golden brown, 30–40 minutes. Add sherry and cook, stirring and scraping the bottom of the skillet, until absorbed, about 1 minute. Transfer to a bowl and set aside. Wipe skillet clean.

Parcook Potatoes:
While onions cook, place sliced potatoes in a medium saucepan with 1 tablespoon salt and enough water to cover by 1 inch. Bring to a boil over high heat and cook until just tender, about 3 minutes. Drain well and place on a paper towel–lined baking sheet. Pat dry and set aside.

Brown Potatoes:
Line a baking sheet with paper towels. Heat 2 tablespoons butter and 1/2 tablespoon oil in the clean skillet over medium-high. Add half of the potatoes in a single layer. Sprinkle with 3/4 teaspoon salt. Cook undisturbed until golden brown on one side, 3–5 minutes. Flip and brown the other side, another 3–5 minutes. Transfer to lined baking sheet. Repeat with remaining butter, oil, potatoes, and salt.

Combine Ingredients:
Return all browned potatoes to skillet. Add caramelized onions, black pepper, and remaining 1/2 teaspoon salt. Stir gently to combine.

Broil and Finish:
Preheat oven broiler with rack 6 inches from heat source. Sprinkle Gruyère evenly over potatoes. Broil for 3–5 minutes, until cheese melts and bubbles. Remove from oven, sprinkle with fresh thyme, and serve hot.