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Grease a 9x13 inch pan with cooking spray.
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In a large bowl, whish together eggs, onion powder, half and half, salt, and pepper until well combined; set aside.
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Layer the bread in a single layer, butter side up in your greased pan. {You may need to cut the bread to fit the pan.}
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Evenly sprinkle the cheese over the bread, followed by the tomatoes, and then the spinach.
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Top it off, by pouring the egg mixture over the casserole.
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Cover and refrigerate overnight. {Or for at least one hour.}
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Bake at 350°F for 35-45 minutes or until center is set.