Cut croissants into varying sizes - ½, ⅓, and bite sized. Place in the bottom of a 9x13" baking dish. Sprinkle with cubed ham, ⅔ of the green onions, and ½ of the white cheddar cheese. Set aside.
Whisk eggs and milk in a medium mixing bowl until combined. Pour over prepared croissant mixture. Top with remaining cheese and green onions. Cover, and chill for 4 hours to overnight.
Preheat oven to 375°F. Bake overnight casserole for 35-40 minutes, or until egg mixture has set and top is golden. Top with everything bagel seasoning. Allow to cool for 5 minutes before slicing and serving.
Notes
Croissants should be slightly stale, or a day old, in order to soak up as much of the egg mixture as possible. If the croissants are fresh, cut them and place them on a baking tray at 250°F for 45 minutes to dry out before using.