1️⃣ Prepare the potatoes: Preheat the oven to 400°F (200°C). Scrub the potatoes clean, dry them thoroughly, and prick each one several times with a fork. Rub them with a little olive oil and sprinkle with salt. Place directly on the oven rack or a baking sheet and bake for 50 to 60 minutes until the skin is crisp and the flesh is tender.

2️⃣ Caramelize the onions: While the potatoes bake, heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the sliced onions and sprinkle with sugar. Cook, stirring occasionally, for 20 to 25 minutes until the onions are deeply golden and caramelized. Add thyme, salt, and pepper, then pour in the broth to deglaze the pan. Let the mixture simmer for 2 to 3 minutes until slightly reduced.

3️⃣ Assemble the potatoes: Remove the baked potatoes from the oven and let them cool slightly. Slice each potato lengthwise down the center without cutting all the way through. Gently fluff the interior with a fork to create space for the filling. Spoon the warm onion mixture into each potato.

4️⃣ Add cheese and bake: Sprinkle a generous amount of shredded Gruyère cheese on top of each filled potato. Return to the oven for about 10 minutes, or until the cheese is melted, bubbling, and slightly browned.

5️⃣ Garnish and serve: Remove the potatoes from the oven and finish with a sprinkle of chopped fresh parsley if using. Serve hot as a savory side or pair with a salad for a comforting main dish.