French Onion Lyonnaise Potatoes

Ingredients

• 3 tablespoons olive oil, divided
• 2 large sweet onions, thinly sliced lengthwise (about 5 1/2 cups)
• 6 teaspoons kosher salt, divided
• 1/4 cup dry sherry
• 1 1/2 pounds Yukon Gold potatoes (about 4 medium), peeled and sliced crosswise into 1/4-inch-thick rounds (about 2 cups)
• 4 tablespoons unsalted butter, divided
• 1/2 teaspoon black pepper
• 4 ounces Gruyère cheese, shredded (about 1 cup)
• 1 teaspoon fresh thyme leaves

Steps

1. Heat 2 tablespoons oil in a large cast-iron skillet over medium. Add onions and 1 teaspoon salt; reduce heat to medium-low and cook, stirring occasionally, until deep golden brown, 30 to 40 minutes. Add sherry; cook, stirring constantly and scraping any browned bits from bottom of skillet, until sherry is fully absorbed, about 1 minute. Transfer onions to a medium heatproof bowl; set aside. Rinse and wipe skillet clean.
2. While onions are cooking, place potatoes and 1 tablespoon salt in a medium saucepan, and fill with enough water to cover potatoes by 1 inch. Bring to a boil over high. Boil over high, undisturbed, until just tender when pierced with a fork, about 3 minutes. Drain well and transfer potatoes to a paper towel–lined baking sheet in a single layer. Pat dry and set aside until ready to use.
3. Line a separate large baking sheet with paper towels; set aside. Heat 2 tablespoons butter and 1/2 tablespoon oil in cleaned skillet over medium-high. Add half of the parcooked potatoes in a single layer; sprinkle with 3/4 teaspoon salt. Cook, undisturbed, until golden brown on bottom, 3 to 5 minutes. Flip and cook, undisturbed, until golden brown on second side, 3 to 5 minutes. Transfer potatoes to prepared baking sheet to drain and wipe skillet clean. Repeat process once more with remaining 2 tablespoons butter, 1/2 tablespoon oil, half parcooked potatoes, and 3/4 teaspoon salt.
4. Return all browned potatoes to cleaned skillet. Add caramelized onions, black pepper, and remaining 1/2 teaspoon salt; stir until incorporated (it is fine if some onions are not completely dispersed).
5. Preheat oven to broil with rack about 6 inches from heat source. Sprinkle potato mixture evenly with Gruyère; broil potatoes until cheese has melted, 3 to 5 minutes. Remove from oven, and sprinkle with thyme. Serve immediately.