Preheat your oven to 425°F (220°C). Line a large rimmed sheet pan with parchment paper for easy cleanup, or lightly oil it.
Make the marinade: In a bowl, whisk balsamic vinegar, olive oil, honey, Dijon, garlic, Italian seasoning, salt, pepper, and a pinch of red pepper flakes. Taste and adjust salt or sweetness.
Prep the chicken: Pat chicken dry.
If using breasts, consider cutting them in half horizontally for even cooking. Add chicken to a bowl or zip-top bag and pour in about half the marinade. Toss to coat and let sit while you prep veggies (10–15 minutes).
For deeper flavor, marinate up to 8 hours in the fridge.
Prep the veggies: Add all chopped vegetables to the sheet pan. Drizzle with half of the remaining marinade (reserve a little for brushing later), season with extra salt and pepper, and toss to coat. Spread into an even layer with space between pieces.
Add the chicken: Nestle the marinated chicken on top of the veggies.
Brush or spoon any marinade from the chicken bowl over the pieces. Discard any marinade that touched raw chicken.
Roast for 20 minutes. Remove the pan, brush the chicken with the last bit of reserved clean marinade, and toss the veggies for even browning.
Return to the oven and roast another 8–12 minutes, until the chicken is cooked through (internal temp of 165°F/74°C) and the veggies have browned edges.
Thighs may need a couple minutes more than breasts.
Optional broil: For extra caramelization, broil on high for 1–2 minutes. Watch closely.
Rest and finish: Let the chicken rest for 5 minutes. Sprinkle with chopped fresh parsley or basil and add a light squeeze of lemon for brightness.
Serve with rice, quinoa, mashed potatoes, or a simple side salad.
Spoon any pan juices over the top for extra flavor.