Preheat oven to 400 degrees. Coat a 9 x 9-inch baking pan with cooking spray.
In a medium bowl, add chopped spinach, onions, and garlic. Cook for 1 minute, in the microwave, to soften.
In a large bowl, mix together frozen hash browns, chicken, cooked spinach, and onions. In a medium bowl, whisk together eggs, ⅔ cup condensed soup, milk, seasoning salt, and black pepper. Pour over potato mixture and toss to mix. Add mixture to baking pan. Be sure to spread the chicken and spinach around the casserole. Cover with foil and bake for 30 minutes. Remove foil, sprinkle with both kinds of cheese. Bake uncovered, an additional 20 minutes until golden brown and cheese is melted.
Let it sit for 10 minutes to set. Cut into 6 slices. Using a spatula, carefully remove each slice.
This casserole freezes great. I like to wrap into individual servings and store in the freezer. Great for a quick meal any day of the week!