Winter Greens Gratin

Preparation

1. Step 1

   Lightly butter a 3-quart baking dish. Working in batches, blanch kale in a pot of boiling lightly salted water until just softened, about 3 minutes. Using a slotted spoon, transfer kale to a large bowl of ice water; let cool, then drain. Squeeze out as much excess water as possible by hand and transfer kale to a work surface.

   Step 2

   Repeat with mustard greens, blanching for 2 minutes per batch. Coarsely chop all greens and combine in a large bowl (you should have 6 cups tightly packed greens). Separate clumps and loosen chopped leaves by hand.

   Step 3

   Heat oil in a 12" skillet over medium heat. Add breadcrumbs and cook, stirring frequently, until golden and crispy, 8-10 minutes. Transfer to a large bowl; stir in Parmesan and 1 teaspoon thyme leaves. DO AHEAD: Greens and breadcrumbs can be made 2 days ahead. Cover greens and chill. Store breadcrumbs airtight at room temperature.

   Step 4

   Melt 1 tablespoon butter in a medium saucepan over medium-high heat. Add shallots; cook, stirring often, until slightly softened and light golden, about 5 minutes. Transfer shallots to bowl with greens. Add cream, milk, garlic, and thyme sprigs to same saucepan; bring to a simmer. Cook until mixture is thickened and reduced to 1 1/2 cups, 10-12 minutes. Discard thyme sprigs and garlic; stir in nutmeg. Season mixture with salt and pepper.

   Step 5

   Pour cream mixture over greens in bowl and toss to evenly coat in sauce; season to taste with salt and pepper. Transfer greens mixture to prepared baking dish; sprinkle Gruyère over. Top with breadcrumbs. Cover dish with foil. DO AHEAD: Gratin can be assembled 1 day ahead.

   Step 6

   Preheat oven to 400°F. Bake until filling is hot, about 25 minutes. Uncover and bake until cheese is melted, edges are bubbling, and breadcrumbs are golden brown, 10-20 minutes longer. Garnish with remaining 1 teaspoon thyme leaves.