Step 1

Place potatoes in a large pot and cover with cold water by 1 inch. Add 2 tablespoons salt and bring to a boil over high. Reduce heat to medium; simmer potatoes until just tender when pierced with a fork, 15 to 20 minutes. Drain; let cool for 15 minutes. Cut into ¾-inch pieces. Transfer to a large bowl and toss with melted butter, several grinds of pepper, and 1 teaspoon salt. Set aside.

Step 2

Preheat oven to 400°F. Place pancetta in a medium skillet. Cook over medium, stirring occasionally, until golden and crispy and fat has rendered, about 15 minutes. Transfer to a bowl using a slotted spoon. Reserve 2 tablespoons drippings in skillet. Add onion and ¼ teaspoon salt to skillet; cook, stirring often, until golden brown, 8 to 10 minutes.

Step 3

Return pancetta to skillet and add wine. Bring to a simmer over medium-high; cook until liquid is mostly absorbed, 5 to 6 minutes. Remove from heat. Stir in cream and remaining ¼ teaspoon salt.

Step 4

Arrange half of potatoes in a broiler-safe 8-inch square baking dish. Top with half of pancetta-onion mixture and half of cheese. Repeat layering with remaining potatoes, pancetta-onion mixture, and cheese.

Step 5

Bake until cheese has melted, about 15 minutes. Increase oven temperature to broil (do not remove casserole from oven). Broil until browned in spots, 2 to 3 minutes.