Preparation Steps

• Preheat the oven to 400°F (200°C). Scrub potatoes, prick with a fork, rub with a little oil and salt, and bake directly on the oven rack until tender, about 45 to 60 minutes. Let cool until safe to handle.
• While potatoes bake, heat butter and olive oil in a large skillet over medium heat. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, until onions are deeply golden and caramelized, about 25 to 30 minutes.
• Pour in the beef broth and Worcestershire sauce, scraping up any browned bits. Simmer until most of the liquid has reduced and the onions are glossy, about 5 minutes. Stir in thyme, then remove from heat.
• When potatoes are cool, cut each in half lengthwise and scoop out the flesh, leaving a thin shell (reserve scooped potato in a bowl). Place skins on a baking sheet, brush inside lightly with oil, and bake skin-side down at 425°F (220°C) for 10 minutes to crisp.
• Mash the reserved potato flesh with 1.0 tbsp butter, 1.0 cup of the shredded Gruyère, salt and pepper. Fold in about two-thirds of the caramelized onions and adjust seasoning.
• Fill each crisped potato skin with the onion-potato-cheese mixture. Top each with remaining shredded Gruyère and a little extra thyme.
• Broil the filled potatoes on high for 2 to 4 minutes, watching closely, until the cheese is melted and golden brown. Remove from oven.
• Serve warm topped with a dollop of sour cream if desired and a sprinkle of extra thyme or chives.