Combine first 7 ingredients in a medium bowl, stirring with a whisk. Add chicken, onion, and cheese, tossing gently.
Place 1 toast slice on each of 4 plates. Top each with 1/2 cup arugula, 1 tomato slice, and about 1/2 cup chicken mixture.
Arugula has a peppery flavor that's more intense than that of most lettuces. It's highly perishable and will only last for a few days after purchase. Store it in the refrigerator inside a perforated plastic bag. Rinse thoroughly, and pat dry before using.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.