Crispy Skillet Latke with Kale

Heat 2 tablespoons oil in a large (12-inch) cast-iron skillet over medium. Add red onion and 1/2 teaspoon salt; cook, stirring occasionally, until softened and lightly browned, about 8 minutes. Add kale and 1 teaspoon water; cook, folding kale into onion mixture, until kale wilts, about 2 minutes. Add vinegar; cook, stirring constantly, until evaporated, about 20 seconds. Remove from heat. Transfer mixture to a small heatproof bowl; set aside. Wipe skillet clean.

Preheat oven to 475°F with rack in middle position. Grate potatoes and yellow onion on large holes of a box grater. Working in batches, place grated vegetable mixture on a clean kitchen towel, and squeeze firmly over sink to remove as much liquid as possible. Transfer mixture to a large bowl. Add eggs, panko, chives, lemon zest, pepper, and remaining 2 teaspoons salt; stir until combined.

Add 1/4 cup oil to cleaned skillet, and swirl to coat. Heat over medium-high until oil shimmers. Spoon potato mixture into skillet (mixture should sizzle on contact). Using a spatula, press mixture into an even layer to cover bottom of skillet. Brush top of latke with remaining 2 tablespoons oil. Bake in preheated oven on middle rack until latke bottom is browned and edges are crisp, about 15 minutes.

Heat broiler to high (do not remove skillet from oven). Broil until top of latke is golden brown in spots, 4 to 5 minutes. Remove from oven; let cool 5 minutes.

Place a large plate on top of skillet, and invert to remove latke. Sprinkle latke lightly with salt. Top with kale mixture and, if desired, dollops of crème fraîche. Garnish with additional chives.