Preheat the oven to 375 degrees F. Spray a 3-quart baking dish with cooking spray.
In a large bowl, combine the evaporated milk, flour, paprika, seasoning salt, nutmeg, cayenne, 3/4 teaspoon kosher salt and 1/2 teaspoon black pepper. Add the potatoes, onions and zucchini and toss to coat, making sure to separate the vegetable slices.
Pour the mixture into the prepared baking dish, distribute the vegetables evenly, and then flatten the top with a rubber spatula. Sprinkle with the Parmesan and bake until crispy on top and the potatoes are tender, 45 minutes to 1 hour.
Let sit for 10 minutes to set. Garnish with green onions before serving.