Preheat the oven to 400°F (200°C). Wash and dry the potatoes, then poke them several times with a fork to allow steam to escape. Rub each potato with a little olive oil and sprinkle with salt. Place the potatoes directly on the oven rack or on a baking sheet, and bake for 50–60 minutes, or until the skins are crisp and the insides are tender.

While the potatoes bake, heat olive oil in a large skillet over medium heat. Add the sliced onions and sprinkle with sugar. Cook, stirring occasionally, for 20–25 minutes, or until the onions become deeply caramelized. Stir in thyme, salt, and pepper, then deglaze the pan with beef broth, allowing it to simmer for another 2–3 minutes until the liquid reduces slightly.

Once the potatoes are cooked, remove them from the oven and let them cool slightly. Slice each potato lengthwise down the center, being careful not to cut all the way through. Gently fluff the insides with a fork to create space for the filling. Spoon the caramelized onion mixture evenly into each potato.

Top each potato with a generous amount of shredded Gruyère cheese. Return them to the oven and bake for an additional 10 minutes, or until the cheese is melted and bubbly.