Ingredients
- 1 tablespoon unsalted butter, plus more for greasing
- 1 tablespoon extra-virgin olive oil
- 1 medium sweet onion, cut into 1/4-inch-thick slices
- Kosher salt and freshly ground pepper
- 1 cup heavy cream
- 1 cup whole milk
- 1 garlic clove, grated
- 2 sprigs fresh thyme
- 2 pounds Yukon Gold potatoes (6-8 medium)
- 1/4 cup fresh Parmigiano-Reggiano cheese, finely grated
- 3 1/2 ounces Gruyère cheese, shredded (1 packed cup)
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