To stuff a chicken breast half, first place it between 2 sheets of heavy-duty plastic wrap.
Pound to a 1/4-inch thickness using a meat mallet or rolling pin. Discard plastic wrap.
For this recipe, top each chicken breast half with 1 slice prosciutto, 2 halves of a slice of Gruyere cheese, and 1/4 cup arugula, leaving a 1/4-inch border around edges.
Fold the chicken breast in half, pinching edges together to seal; sprinkle with salt and pepper.
You can prepare the chicken up to this point up to a day ahead and refrigerate. Or go ahead and immediately proceed with cooking the chicken according to the recipe directions.