Follow these step-by-step instructions for a foolproof method to achieve perfectly juicy chicken and tender-crisp roasted vegetables every time. The key is in the preparation and the roasting technique.

Step 1: Preheat Your Oven and Prepare Baking Sheets
First things first, position your oven racks in the upper-middle and lower-middle positions. Preheat the oven to 400°F (200°C). This high heat is crucial for getting a good roast on the vegetables and a nice sear on the chicken, rather than just steaming them. Line two large, rimmed baking sheets with parchment paper for easy cleanup and to prevent sticking.

Step 2: Marinate the Chicken
In a large mixing bowl, combine the cubed chicken with the ¼ cup of extra virgin olive oil, minced garlic, dried oregano, dried rosemary, dried thyme, sea salt, and black pepper. Use a spoon or your hands to toss everything together until each piece of chicken is thoroughly coated in the herb and garlic mixture. Set the bowl aside to marinate for at least 15-20 minutes at room temperature. This brief period allows the flavors to begin penetrating the meat. If you have more time, you can cover it and let it marinate in the refrigerator for up to 4 hours for even deeper flavor.

Step 3: Prepare the Vegetables
While the chicken is marinating, place all the chopped vegetables—broccoli florets, bell peppers, and red onion—into another large bowl (or you can do this directly on one of the prepared baking sheets). Drizzle them with 2 tablespoons of olive oil and season generously with salt and pepper. Toss well until every vegetable is lightly coated in oil.

Step 4: Arrange on Baking Sheets
This is the most critical step for achieving perfectly roasted results. Spread the prepared vegetables in a single, even layer on one of the baking sheets. Then, arrange the marinated chicken pieces in a single, even layer on the second baking sheet. It is vital not to overcrowd the pans. Crowding the ingredients will cause them to steam instead of roast, resulting in soggy vegetables and pale chicken. Using two separate sheets ensures that there is enough space for the hot air to circulate around each piece, promoting browning and caramelization.

Step 5: Roast to Perfection
Place both baking sheets in the preheated oven. Place the vegetables on the upper-middle rack and the chicken on the lower-middle rack. Roast for 20-25 minutes. About halfway through the cooking time (around the 12-minute mark), remove the pans from the oven, toss the vegetables, and flip the chicken pieces over. This ensures even cooking and browning on all sides.

Step 6: Check for Doneness
The meal is ready when the vegetables are tender-crisp and have nicely browned, caramelized edges. The chicken should be cooked through, opaque, and juicy. The most reliable way to check the chicken is with a meat thermometer; it should register an internal temperature of 165°F (74°C) at its thickest point.

Step 7: Final Touches and Assembly
Once cooked, remove both pans from the oven. Squeeze the juice of half a lemon over the hot chicken and the other half over the roasted vegetables. This immediate hit of acidity brightens all the flavors. Sprinkle both the chicken and vegetables with the chopped fresh parsley. To assemble the bowls, start with your desired base (like quinoa or brown rice), then top with a generous portion of the garlic herb chicken and roasted vegetables.