Preheat the oven to 400°F (200°C). Cut the chicken breasts into 1-inch pieces, and place them in a large mixing bowl.
Add the bell pepper, onion, artichoke hearts, spinach, and parmesan cheese (if using) to the mixing bowl on top of the chicken.
In a separate small mixing bowl, stir together the mayonnaise, garlic, lemon zest and juice, oregano, thyme, salt, and pepper.
Pour the dressing on top of the chicken and veggies and gently stir until well combined.
Place the mixture into a 9x13-inch baking pan. Bake uncovered for 25 to 30 minutes or until the chicken is cooked through.