Spray 9 x 13 baking dish with nonstick cooking spray or lightly coat with butter.
Combine ham, cheese, onions, and croissants in a medium mixing bowl. Arrange in prepared baking dish. Whisk together milk, cream, and eggs and pour over the croissant mixture. Cover and chill overnight.
Preheat oven to 350º F. Uncover the casserole and sprinkle with the Cheddar cheese. Bake until golden brown and cooked throughout, about 45 minutes. Remove from the oven and let stand for 5 minutes before serving.
Make-Ahead: Cover and store the Ham and Cheese Croissant Casserole in the refrigerator for up to 3 days. You may also assemble the casserole without baking and cover and store in the refrigerator for up to 3 days. Then, remove from the refrigerator, uncover and bake as directed.