Preheat the oven to 350˚F. Butter an 8-inch baking dish.
Cook the rice according to package instructions and set aside.
Wash the kale, strip the leaves from the stems, and cut the leaves into 1 inch strips. Bring a large pot of salted water to a boil. Blanch the kale for 2 minutes. Drain the kale and rinse to cool, squeezing out any excess water. Set aside.
Heat the olive oil in a large saucepan over medium-high heat. Add the onion and cook for 4 minutes until it begins to brown. Add the chicken stock and boil for 3 minutes until the liquid is reduced to less than 1/4 cup. Reduce the heat to medium-low and stir in the milk along with the salt, pepper, nutmeg, rice, and kale. Remove from the heat and stir in the egg, yolk, and all but 1-1/2 cups of the cheese, mixing thoroughly.
Spread the mixture in the baking dish and top with the reserved 1/4 cup cheese and the bread crumbs. Bake for about 40 minutes or until the gratin is golden brown on top and bubbling around the edges.