INGREDIENTS
4 medium russet potatoes (8 to 10 ounces each)
1 pound yellow onions (about 1 large or 2 medium)
1/2 small bunch fresh thyme (about 10 sprigs), plus more for serving
1 clove garlic
2 ounces Gruyere cheese
4 tablespoons unsalted butter, divided
1/2 teaspoon kosher salt, plus more for potatoes
1/2 teaspoon freshly ground black pepper, plus more for potatoes
1 tablespoon all-purpose flour
1 cup beef broth
1/2 teaspoon balsamic or sherry vinegar
INSTRUCTIONS
1. Place one oven rack 6 to 8 inches from the broiler and a second rack in the middle of the oven. Heat oven to 400°F.
2. Prepare 4 medium russet potatoes: Rub potatoes, then cut crosswise about 1/4 inch deep into each potato, leaving a half-inch border. Place the potatoes cut side up directly on the middle rack of the oven. Bake until very crispy and crunchy on the outside, about 2 hours.
3. When the potatoes have about 30 minutes left, cut them in half and thinly slice 1 pound of yellow onions (about 4 cups). Gather the leaves of 1/2 small bunch of fresh thyme to 1 tablespoon. Chop 1 clove of garlic. Grate 2 ounces of Gruyere cheese on the large holes of a box grater (filling 1/2 cup).
4. Melt 2 tablespoons unsalted butter in a 12-inch heavy-bottomed cast iron skillet or other heavy-bottomed grill-safe skillet over medium heat. Add the onion, thyme, 1/2 teaspoon kosher salt, and 1/2 teaspoon black pepper. Cook, stirring frequently with a wooden spoon and reducing the heat if the onions begin to brown too quickly, until they have reduced by more than half, are very soft and a deep golden color, 25 to 30 minutes.
5. Add the garlic and 1 tablespoon all-purpose flour and stir until the flour is completely incorporated, leaves a film on the bottom of the pan, and begins to brown, about 1 minute. Add 1 cup beef broth and scrape up any browned bits from the bottom of the pan. Increase the heat to medium if you lowered it while cooking the onions. Bring to a boil. Simmer until sauce is thick enough to coat the back of a spoon, about 3 minutes.
6. Remove the pan from the heat. Add 1/2 teaspoon balsamic or sherry vinegar and stir to combine. Taste and season with more kosher salt and black pepper as needed.
7. Carefully remove the potatoes from the oven with tongs and place them on a baking sheet. Carefully cut deeper into the cuts you initially made. Gently push the two opposite ends of each potato toward each other to open them, releasing steam, and exposing some of the flesh.
8. Return the potatoes to the bottom rack of the oven and bake for 10 more minutes. Take them out of the oven. Push the two opposite ends of each potato toward each other again to open them completely. Cut the remaining 2 tablespoons of unsalted butter into 4 pieces and add a piece to each potato. Add a pinch of kosher salt and black pepper to each potato. Evenly divide onion mixture among potatoes (about 1/4 cup per potato) and top evenly with cheese.
9. Turn oven on to broil. Transfer the baking sheet to the top rack and broil until the cheese is melted, bubbling, and browned in spots, 3 to 4 minutes. Garnish with more fresh thyme leaves if desired.
RECIPE NOTES
Do Ahead: The onion mixture can be made up to 2 days in advance and refrigerated in an airtight container. Let it sit at room temperature for 30 minutes or reheat in the microwave for 30 to 40 seconds before topping the potatoes.
Storage: Leftovers can be refrigerated in an airtight container for up to 4 days.