To a large bowl add in the eggs and milk whisk well to break the yolks. Stir in the onion, spinach, and seasonings stir well. Grease a 9x13 baking dish with non-stick spray, pour the egg mixture in. Bake for 25-30 minutes or until the egg has set - if meal prepping wait until the egg has completely cooled down to assemble the sandwiches.