Preheat the oven to 350 degrees F. Spray a 9x13 inch baking dish with nonstick spray.
Tear the kale into pieces and place it in the baking dish. It may be overflowing, but that’s good! It will cook way down in the oven. Toss the shallot into the kale and season it all with the salt and pepper, tossing well.
Toss the cheeses together in a bowl. Add almost all of (about three quarters) the cheese to the kale, tossing it well to combine.
In a large bowl or measuring cup, whisk together the garlic, cream, mustard and nutmeg. Pour the cream mixture over top of the kale. Cover with the remaining cheese. Sprinkle the breadcrumbs on top.
Bake the gratin for 45 minutes, until the top is golden and crisp and the edges are bubbly. Serve immediately!
Note: this can be made ahead of time, but it is best to reheat right before serving!