You can prepare this casserole and store it in the fridge overnight. The next morning, let the casserole stand at room temperature while the oven heats. Then bake it as directed.
Preheat the oven to 375 F.
Coat a 7-by-11-inch glass baking dish or a 2-quart casserole with cooking spray.
Place the spinach in a strainer. Press with the back of a spatula to remove excess liquid. Set aside.
In a medium bowl, whisk together the eggs, egg whites and milk. Add dry mustard, rosemary, spice blend and pepper. Whisk to combine.
Toss the spinach, bread, onion and red pepper in a large bowl. Add the egg mixture and toss to coat.
Transfer to the prepared baking dish. Push down to compact. Cover with foil.
Bake for 30 minutes or until the eggs have set. Uncover and then top with cheese. Continue baking for another 15 minutes or until the top is lightly browned.
Transfer to a wire rack. Cool for 10 minutes before serving.
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