Step 1: Preheat the Oven

• Preheat your oven to 400°F (200°C).
• Arrange oven racks with one in the middle and another 6–8 inches below the broiler.

Step 2: Prepare the Potatoes

• Scrub the potatoes clean and pat them dry.
• Cut a cross shape, about ¼-inch deep, into the tops of the potatoes.
• Place potatoes directly on the middle oven rack and bake for 2 hours, until skins are crispy and interiors are fluffy.

Step 3: Caramelize the Onions

• Melt 2 tablespoons of butter in a skillet over medium heat.
• Add sliced onions, thyme, salt, and pepper. Cook, stirring frequently, for 25–30 minutes until onions are deeply caramelized.

Step 4: Make the Onion Sauce

• Stir in minced garlic and cook for 1 minute.
• Sprinkle flour over the onions and stir to form a paste.
• Gradually add beef broth, scraping up browned bits, and simmer for 3 minutes until thickened.
• Remove from heat, stir in balsamic vinegar, and adjust seasoning to taste.

Step 5: Finish the Potatoes

• Remove potatoes from the oven and deepen the cross cuts.
• Squeeze the sides gently to open them up, then return to the oven for 10 more minutes.

Step 6: Assemble the Topping

• Place the potatoes on a baking sheet.
• Add a small pat of butter to each potato and season with salt and pepper.
• Spoon the onion mixture evenly onto each potato and top with grated Gruyère cheese.

Step 7: Broil

• Switch the oven to broil and place the baking sheet on the top rack.
• Broil for 3–4 minutes until the cheese melts and turns golden brown.