Preheat the oven to 400 degrees F. Prepare a large cookie sheet (with sides) with a light coating of cooking oil spray. Cook the onion and mushrooms until onion is soft and water has cooked out of mushrooms. Remove from heat. Squeeze all the water from the thawed spinach. (I wring it out between paper towels.) Add the spinach, bacon and water chestnuts to the mushrooms and onions. Beat the eggs in a large bowl and season with salt and pepper. Add cooled spinach/mushroom mixture and blend into the eggs. Pour the egg mixture into the cookie sheet and evenly spread into the corners. Bake at 400 degrees for 20 minutes or until set and beginning to brown on top. Cut into squares to serve. Freezes well in as single layer of cut squares in a zip-lock freezer bag. Microwave to reheat.