Heat oil in a large nonstick skillet. Sprinkle salt and pepper on chicken and cook over medium-high heat for 3 - 4 minutes on each side or until done. When cooked thoroughly, move to a cutting board and cool slightly. Cut chicken into thin slices. Slice bread to make four sandwiches.
Preheat Panini maker. Brush outside slices of bread with olive oil. Arrange chicken slices evenly over 4 slices of bread. Top chicken with tomato slices, onion, roasted red pepper and cheese. Spread pesto on top slices of bread and place on sandwiches. Place into Panini maker and toast until bread is browned and cheese is melted about 2 minutes.
If you don't own a Panini maker, you can heat this sandwich in a skillet or under the broiler.