Creating the Ina Garten Overnight Breakfast Casserole is straightforward if you follow these steps:

1. Prepare the Dish: Grease a 3-quart baking dish or a large casserole dish.
2. Cube the Bread: Place the bread cubes into the dish evenly.
3. Whisk the Eggs: In a large bowl, whisk together the eggs, milk, heavy cream, Dijon mustard, salt, and black pepper until well combined.
4. Add Cheese and Meat: Stir in the shredded cheddar cheese, diced ham, and chopped chives.
5. Combine Mixtures: Pour the egg mixture over the bread cubes, ensuring all pieces are soaked. Press down gently to help the bread absorb the liquid.
6. Cover and Chill: Cover the dish tightly with plastic wrap and refrigerate overnight.
7. Preheat Oven: The next morning, preheat your oven to 350°F (175°C).
8. Bake: Remove the plastic wrap and bake the casserole for 45-50 minutes or until it’s golden brown and set in the center.
9. Cool Slightly: Let it cool for about 10 minutes before serving. This allows it to set further.
10. Serve: Garnish with additional chives or cheese if desired before serving.

These steps will guide you in making this incredible breakfast casserole with ease.