Prep. Spray 9 x 13 baking dish with nonstick cooking spray or lightly coat with butter.
Make and chill. 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.
Bake and serve. 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.
Notes
Make Ahead and Storage Tips:
To 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. To store. Once baked, allow to cool, wrap well with plastic wrap or foil, and store in the refrigerator for up to 3 days. Reheat and serve.To store leftovers. Allow leftovers to cool completely. Then store in an airtight container in the refrigerator for up to 3 days. Reheat and serve.I do not recommend freezing this breakfast casserole. Many times, cream and dairy-based recipes do not freeze well, as they can sometimes break when frozen.