Potato & Celery Root Gratin with Leeks

INGREDIENTS

• 2 pounds russet potatoes, peeled, very thinly sliced crosswise (1/8" thick)
• Freshly ground black pepper
• 3 cups heavy cream
• 2 tablespoons (1/4 stick) unsalted butter, divided
• 2 garlic cloves, peeled
• 3 leeks, white and pale-green parts only, halved lengthwise, thinly sliced crosswise
• 1 sprig thyme plus 3 teaspoons fresh thyme leaves, divided
• 2 cups grated Gruyère
• 1 pound celery root, peeled, very thinly sliced crosswise (1/8" thick)
• Kosher salt