This dish is all the best of potatoes au gratin packed into the convenience of a sheet pan supper. There’s plenty of crispy, cheese topping that’s got a nice element of fresh thyme flavor, the corners and edges maintain a nice crisp, and the potatoes are perfectly cooked in the center. The ham and peas make this a full meal, but not so much that anyone will feel like this is anything but comfort food. When you’re adding the cheese sauce, shake the baking sheet a bit to make sure the sauce permeates the potato layers and settles in—this will result in perfect distribution and prevent any overflowing in the oven.

Ingredients

Ingredient Checklist

• ¼ cup unsalted butter, divided
• 2 garlic cloves, pressed
• 3 tablespoons all-purpose flour
• 3 cups heavy cream
• 11 ounces Gruyère cheese, shredded (about 2 3/4 cups), divided
• ½ teaspoon black pepper
• 2 ½ teaspoons kosher salt, divided
• 1 (13-oz.) pkg. frozen sweet peas, thawed (about 1 1/2 cups)
• 2 ½ pounds Yukon Gold potatoes, cut into 1/8-in.-thick slices
• 1 pound thinly sliced deli ham, chopped
• 2 cups panko (Japanese-style breadcrumbs)
• 1 tablespoon fresh thyme leaves

Directions

• Step 1Preheat oven to 450°F. Grease an 18- x 13-inch rimmed baking sheet with 1 tablespoon of the butter.
• Step 2Melt remaining 3 tablespoons butter in a saucepan over medium. Add garlic; cook, stirring often, until fragrant, about 1 minute. Whisk in flour; cook, whisking constantly, 1 minute. Whisk in cream, and cook, stirring constantly, until slightly thickened, about 3 minutes. Remove from heat; add 2 cups of the cheese, and stir in pepper and 2 teaspoons of the salt. Stir until cheese melts and sauce is smooth; stir in peas.
• Step 3Spread half of potato slices and half of ham on prepared baking sheet. Spread half of cheese sauce over potatoes and ham. Repeat layers once with remaining potatoes, ham, and cheese sauce. Cover baking sheet with aluminum foil, and bake in preheated oven until potatoes are tender, about 40 minutes.
• Step 4Stir together panko, thyme, and remaining 3/4 cup cheese in a small bowl. Remove foil, and sprinkle panko topping over potato mixture. Return to oven, and bake at 450°F until panko topping is golden brown, 20 to 25 minutes. 

Original Recipe here