Make-Ahead Mashed Potato Casserole

Bahan-bahan (8 porsi)

• 5 pounds (about 12 medium-sized) Yukon gold potatoes, peeled and quartered
• 5 cups half-and-half
• 8 cloves garlic, peeled and gently smashed
• 4 sprigs fresh thyme
• 2 bay leaves
• 10 tablespoons butter, divided
• Kosher salt and black pepper
• 5 large egg yolks and 1 egg white, divided
• Oil
• 1 bunch fresh sage leaves
• Coarse salt, to top