Pound each chicken breast to uniform thickness and slice in half to yield two pieces of chicken that each fit on a bun. Season with salt and pepper.
Grill 5 minutes per side, until internal temperature is 165 degrees. Grill buns if desired. Add shredded cheese to chicken and let melt before removing from grill.
Spread a thin layer of ranch dressing on the top of each bun.
Place chicken breasts on bun, top with avocado, tomato, and spinach.
Recipe by Supper for a Steal at http://www.supperforasteal.com/ranch-grilled-chicken-sandwich/