Arrange muffin halves, cut sides up, on a baking sheet. Spread 1 teaspoon mustard over each of 4 halves; top each with 1/2 cup turkey and one-fourth of onion. Divide cheese evenly among the remaining 4 halves. Broil 2 minutes or until cheese melts and turkey is warm. Top turkey halves evenly with salt, pepper, and arugula. Top each turkey half with 1 cheese muffin half.