Preheat the oven to 375F
Heat a cast-iron skillet (or another deep ovenproof skillet) over medium heat. Once it’s hot, add about 1 T oil followed by the onion. Season with salt and cook over medium heat for about 10 minutes, stirring occasionally, until tender. Transfer to a medium mixing bowl.
Add half of the kale to the still-hot pan, season with a pinch of salt, and cook, stirring occasionally, until it wilts and any juices evaporate, about 3 minutes. Repeat with the remaining kale. Transfer to the mixing bowl, remove the pan from the heat, and do not wash the pan.
Add the heavy cream, sweet potatoes (thinly sliced), sage, cheese, garlic, pepper, and ½ tsp of salt to the mixing bowl and stir together.
Scoop the mixture into the pan; use your hands to gently tamp it down so it’s an even, compact layer and as submerged as possible in the cream. If it’s extremely full, be sure to place a baking sheet under it to catch any drips.
Put the bread crumbs in a small mixing bowl, drizzle with 1 tsp of olive oil and a pinch of salt, mix to coat, and sprinkle on top of the gratin.
Bake for about 60-75 minutes, until the whole thing is golden brown and bubbly. Let cool slightly before serving.
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