The Best Potatoes au Gratin Ever

Ingredients

• 5 tablespoons unsalted butter, divided
• 1 sweet onion, thinly sliced
• 3 garlic cloves, minced
• 1 tablespoon grainy mustard
• 2 cups half-and-half
• 2 pounds potatoes, peeled and thinly sliced
• Salt and freshly ground black pepper
• ½ teaspoon freshly grated nutmeg
• ¾ cup grated Gruyère cheese