||Preheat grill to high. Brush chicken, bell pepper and red onion with olive oil. Sprinkle with salt and pepper.
||Place chicken and vegetables on grill. Grill chicken 4-5 minutes per side until internal temperature is 170°F. Grill vegetables until tender-crisp, turning once or twice.
||Meanwhile, combine pesto and olives in a small bowl. Slice focaccia into 4 pieces; cut each piece (or each roll) horizontally to make a sandwich. Spread pesto mixture on cut sides of focaccia.
||Slice chicken crosswise 1/8-inch thick. Divide chicken, peppers, onion and cheese evenly between 4 sandwiches. Place top on sandwiches, pesto side down.
||Return sandwiches to grill and reduce heat to medium. Place a heavy cast-iron skillet (or oven-proof skillet weighted with a brick) on top of sandwiches. Close grill lid. Grill 2-3 minutes until bread is crisp; remove skillet, turn sandwiches and repeat pressing with skillet. (May need to grill sandwiches 2 at a time to fit under pan.) Alternatively, press sandwiches in preheated panini grill or sandwich press. Serve warm.