Preheat oven to 400 degrees.

Heat butter in large sauté pan over medium-high heat. Add onion and cook until soft and translucent. Reduce heat and pour kale over onions. Cover pan with lid and cook until kale is reduced and wilted, stirring occasionally. This process should take about 10 minutes, and kale should reduce to about 2 cups. Pour contents of pan into a strainer and allow to drain well while preparing other ingredients. (Kale must be very well drained – squeeze as much liquid out of it as possible while it is in the strainer.)

In the same pan used for the onions and kale (no need to wash it), heat milk and cream over medium heat until they begin to boil. Remove from heat and add nutmeg and blue cheese, whisking to combine.

In a well greased 13×9-inch baking dish, assemble the gratin. First, line the bottom with ¼ of the sweet potatoes. Top with 1/3 of the kale, followed by 1/3 of the milk. Repeat layers, ending with a layer of sweet potatoes.

Cover gratin tightly with foil and bake until liquid is bubbling and potatoes are tender, 45-50 minutes. Remove foil and sprinkle parmesan cheese and panko evenly over gratin. Return to oven, uncovered, and bake another 10-12 minutes, until topping begins to brown. Allow dish to cool at room temperature for 10 minutes before slicing and serving warm.