Preheat oven to 425 F. Line one large or two small baking sheets with foil for easy cleanup. Place empty sheets into preheating oven.
Peel sweet potato and cut into large wedges.
Peel parsnips and cut in half crosswise. Cut thick end in half lengthwise.
Peel beets and cut into large chunks.
Peel onion and cut into large chunks.
Peel carrots and cut in half crosswise. Cut thick end in half lengthwise if necessary.
Trim ends of Brussels sprouts and cut any large ones in half.
Combine herbs and seasonings in a small bowl.
Toss chicken thighs with a tablespoon of the oil and half of the seasoning mixture.
Place chicken pieces onto hot baking sheet, well spaced out. Bake for 10 minutes.
Toss vegetables with a tablespoon of oil and remaining seasoning mixture.
Add them to the baking sheets with the chicken. Spread them out in a single layer with as much space between pieces as possible. Bake for another 10 minutes.
Tuck a whole lemon into a corner of one baking sheet and continue to bake for another 10 minutes or until vegetables are done.
Cut lemon in half crosswise and squeeze the juice over the meat and vegetables.