Preheat the oven to 425°F (220°C).
In a large mixing bowl, combine the olive oil, Italian seasoning, garlic powder, salt, black pepper, and red pepper flakes (if using). Stir to create a marinade.
Add the chicken thighs to the bowl and coat them thoroughly with the marinade.
Chop the mixed vegetables into bite-sized pieces and place them in a separate bowl. Toss them with a little olive oil, salt, and pepper.
Arrange the marinated chicken thighs on one side of the sheet pan and spread the seasoned vegetables on the other side.
Place the sheet pan in the preheated oven and bake for 30 minutes, or until the chicken is cooked through and has an internal temperature of 165°F (74°C) and the vegetables are tender and slightly caramelized.
Remove from the oven and let rest for 5 minutes. Garnish with fresh parsley if desired.
Serve warm and enjoy your delicious Italian seasoned chicken and vegetables!