Mild, buttery fontina cheese melts to flavor the tomato and onion stuffed inside these grilled chicken breasts.
Cut each cutlet in half lengthwise.
Place egg and breadcrumbs in separate shallow glass dishes. Dip chicken piece first in egg, then breadcrumbs to coat completely.
Place 1 piece of cheese, 1 tomato wedge and 1 onion wedge on top of 1 chicken piece. Roll up chicken around the filling. Tie chicken securely in several places with kitchen string. Repeat with remaining chicken pieces.
Meanwhile, heat gas grill to medium-high heat or prepare charcoal grill with medium-hot coals. Grill chicken pieces about 10 minutes per side, or until instant-read thermometer inserted into chicken registers 160°F.
Remove the kitchen string and serve immediately with steamed green beans, if desired.