A strata -- literally, "layers" -- is a savory bread casserole. Essentially, you pour an egg mixture over bread and cheese and let soak in overnight.
Butter a 9-by-13-inch baking dish. Arrange half of the bread slices in the bottom of the dish. Sprinkle with half of the ham, cheese, and chives. Add another layer of bread and cover it with the remaining ham, cheese, and chives. Arrange the tomato slices on top.
In a large bowl, whisk the eggs, milk, mustard, salt, and pepper until well combined. Pour mixture over the bread and cheese layers. Cover and refrigerate overnight.
Preheat oven to 350 degrees. Let strata come to room temperature (about 25 minutes). Bake strata, uncovered, until golden and fluffy and a knife inserted comes out clean (60 to 70 minutes). Let stand 20 minutes before serving.