Preheat the oven to 350°F and grease a 9- by 13-inch casserole dish with the tablespoon of butter.

In a medium saucepan combine the chicken stock and the onion and bring the stock to a boil. Continue to cook until stock is reduced to ½ cup, about 10 minutes. Add the milk, salt, black pepper and cayenne to the saucepan and bring the milk just to a simmer. Remove from the heat and remove the onion (onion may be discarded). In a small microwavable bowl, melt the remaining stick of butter and then whisk in the flour until smooth. Whisk the butter-flour mixture into the hot milk and set aside.

In a large mixing bowl, whisk the egg and egg yolk until thoroughly combined. Add the spinach, white rice, ½ cup of the Parmesan, wild rice, feta cheese, walnuts and green onions and stir to blend. Spoon the spinach mixture into the buttered casserole dish and then pour the hot milk mixture evenly over the top of the spinach. Using the back of a spoon, spread the sauce so that it evenly covers the spinach mixture. Sprinkle the remaining ¼ cup of Parmesan over the top and bake in the center of the oven until the casserole is set and lightly golden on top, 30 to 35 minutes. Serve hot or warm.