Ingredients
2 tablespoons unsalted butter
2 large yellow onions, thinly sliced
1 teaspoon granulated sugar (optional, for faster caramelizing)
1 tablespoon olive oil (optional, if onions look dry)
2 cups heavy cream
1 cup finely grated Parmesan cheese, plus 2 tablespoons for topping
1 1/2 teaspoons kosher salt, divided
1 teaspoon freshly ground black pepper
2 teaspoons fresh thyme leaves, chopped (or 1 teaspoon dried thyme)
3 pounds russet or Yukon Gold potatoes, peeled (if desired) and thinly sliced
1 tablespoon unsalted butter or cooking spray, for greasing the glass casserole dish
