Well, it doesn't get any easier than this. Roast beef and Cheddar are a classic combination. I guess you could call this a healthy version since it uses whole-grain bread and sprouts. Feel free to substitute whatever greens you like.
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Spread the mustard on both slices of bread. Layer the roast beef on one slice and top with the Cheddar, sprouts, and the other slice of bread. Enjoy.