Potato and Caramelized Onion Gratin - Draeger's Market

Ingredients

• 1 tablespoon unsalted butter, softened
• 6 pounds Yukon Gold potatoes, peeled and cut into ¼-inch thick slices
• 1 tablespoon kosher salt, plus more for seasoning
• 2 tablespoons extra virgin olive oil
• 1 pound yellow onions, thinly sliced
• 2 cups grated Gruyere cheese (about 8 ounces)
• 1 cup grated Parmesan cheese
• 1 teaspoon grated nutmeg
• 1 ¼ cups heavy cream
• Freshly ground black pepper