French Onion Potato Gratin

Adjust oven rack to middle position and heat oven to 400 degrees. Combine panko and 3 tablespoons melted butter in bowl. Stir in Gruyère, thyme leaves, and ¼ teaspoon pepper; set aside. Brush 13 by 9-inch baking dish with remaining 1 tablespoon melted butter.

Melt remaining 4 tablespoons butter in Dutch oven over medium-high heat. Add onions, water, sugar, 1 teaspoon salt, and thyme sprigs. Cover and cook, stirring occasionally, until water has evaporated and onions are softened, 12 to 15 minutes. Uncover and continue to cook, stirring often and scraping up any browned bits, until onions are well browned, 12 to 15 minutes. Stir in garlic and cook until fragrant, about 30 seconds. Off heat, discard thyme sprigs.

Stir in broth, sherry, Worcestershire, mustard, remaining ½ teaspoon pepper, and remaining 1 teaspoon salt, scraping up any browned bits, and bring to simmer over high heat. Off heat, stir in potatoes until well combined. Transfer potato mixture to prepared dish and press potatoes into even layer.

Cover dish tightly with aluminum foil, transfer to oven, and bake for 45 minutes. Remove foil and continue to bake until top of potato mixture is spotty brown, potatoes are very tender when pierced with paring knife, and liquid is mostly absorbed, about 30 minutes longer. Remove dish from oven and sprinkle evenly with Gruyère mixture. Return dish to oven and bake until Gruyère mixture is bubbly and spotty brown, 12 to 14 minutes. Let cool for 30 minutes. Serve.