Winter Greens Gratin

INGREDIENTS

• 1 tablespoon unsalted butter plus more for dish
• 2 pounds kale, center ribs and stems removed, torn into large pieces, or two 10-ounce bags trimmed, chopped kale (about 24 cups)
• Kosher salt
• 2 1/2 pounds mustard greens, center ribs and stems removed, torn into large pieces
• 1/4 cup extra-virgin olive oil
• 3 cups coarse fresh breadcrumbs
• 1/2 cup finely grated Parmesan
• 2 teaspoons fresh thyme leaves, divided, plus 7 sprigs thyme
• 2 shallots, sliced into 1/4"-thick rounds (about 1 cup)
• 1 cup heavy cream
• 1 cup whole milk
• 7 garlic cloves, smashed
• 1/8 teaspoon freshly ground nutmeg
• Freshly ground black pepper
• 1 cup coarsely grated Gruyère