Step 1: Cut chicken breasts into uniform 1-inch cubes and place them in a large bowl.
Step 2: Slice bell peppers into 1/2-inch strips, zucchini into 1/2-inch half-moons, and thinly slice the red onion. Add all vegetables to the bowl with the chicken.
Step 3: In a small bowl, whisk together 2 tablespoons of olive oil, minced garlic, dried oregano, dried thyme, dried rosemary, salt, and black pepper. Pour the marinade over the chicken and vegetables. Toss well to coat everything evenly. Marinate for 15-20 minutes at room temperature, or up to 2 hours in the refrigerator.
Step 4: Preheat your Blackstone griddle to medium-high heat. Once hot, add the remaining 1 tablespoon of olive oil and spread it across the surface.
Step 5: Spread the marinated chicken and vegetables in a single layer over the hot griddle. Avoid overcrowding; cook in batches if necessary to ensure proper searing.
Step 6: Cook undisturbed for 4-5 minutes to allow a good sear and caramelization to develop on the bottom.
Step 7: Flip and stir the chicken and vegetables using a griddle spatula. Continue cooking for another 10-15 minutes, stirring every few minutes, until the chicken is cooked through (internal temperature reaches 165°F or 74°C) and the vegetables are tender-crisp and browned.
Step 8: Transfer the cooked chicken and vegetables to a serving platter. Garnish with fresh chopped parsley, if desired, and serve immediately.