2 large bunches flat-leaf or Lacinato kale, stems removed and leaves chopped (about 8 cups chopped)
1 1/2 cups whole milk
3/4 cup low-sodium vegetable broth
1/4 cup fresh sage leaves
2 tablespoons unsalted butter
1 tablespoon olive oil
1 medium onion, diced
2 cloves garlic, minced
2 teaspoons fresh thyme leaves
1/4 teaspoon ground nutmeg
1 teaspoon kosher salt
1/4 teaspoon freshly ground black pepper
1/4 cup all-purpose flour
2 tablespoons apple cider vinegar
4 cups cooked wild rice
2 cups shredded Gruyère cheese, divided
Bring a large pot of heavily salted water to a boil over medium-high heat. Meanwhile, arrange a rack in the middle of the oven and heat to 375°F. Coat a 9x13-inch baking dish with butter or cooking spray; set aside.
Blanch the kale in the boiling water until the leaves are just softened, 1 to 2 minutes. Drain well and set aside.
Place the milk, vegetable broth, and sage leaves in a small saucepan and warm over low heat; keep warm.
Heat the butter and olive oil in a large, high-sided skillet over medium heat until the oil is shimmering. Add the onion and cook, stirring occasionally, until soft, 3 to 4 minutes. Stir in the garlic, thyme, nutmeg, salt, and pepper, and cook for 1 minute. Reduce the heat to low, stir in the flour, and cook for 1 minute more.
Remove and discard the sage leaves from the warmed milk and broth, then gradually whisk the liquid into the skillet until smooth. Stir in the apple cider vinegar and continue cooking until the liquid is slightly thickened, 1 to 2 minutes. Remove from the heat.
Stir in the blanched kale, cooked wild rice, and 1 cup of the cheese. Transfer the mixture to the prepared baking dish in an even layer, then sprinkle with the remaining 1 cup of cheese.
Bake uncovered until heated through and the cheese is melted, 20 to 25 minutes. Remove from the oven and let it stand for about 5 minutes before serving.