Preheat. Heat oven to 425F. Line a large sheet pan with parchment (or lightly oil).
Season chicken. In a bowl, toss chicken with 1 tablespoon olive oil, Italian seasoning, garlic powder, paprika, salt, pepper, Dijon, and lemon juice (if using).
Season veggies. In a second bowl, toss broccoli, bell pepper, onion, and zucchini with remaining 1 tablespoon olive oil and a pinch of salt and pepper.
Arrange. Spread veggies on the sheet pan in an even layer. Nestle chicken among the veggies, leaving space between pieces.
Roast. Roast 18 to 22 minutes (thighs) or 15 to 18 minutes (breast cutlets), then add cherry tomatoes if using. Continue roasting 5 to 8 minutes more, until chicken is cooked through (165F) and veggies are browned and tender.
Finish and serve. Rest chicken 5 minutes. Finish with herbs, lemon, and Parmesan if you want. Serve hot.