Preheat the oven to 350°F.
Add the boneless, skinless chicken thighs to a large bowl and season them with salt, pepper, garlic powder, onion powder, rosemary, thyme, and paprika. Drizzle the chicken with olive oil and toss everything together until the chicken is completely coated in the seasonings. Transfer the seasoned chicken thighs to a 9 x 13" baking dish, arranging them in a single layer.
Peel the carrots, cut them into approximately 2" chunks, and nestle them in between the chicken thighs. Sprinkle the diced onion evenly over the top of the chicken and vegetables.
Pour the chicken broth into the baking dish, cover tightly with aluminum foil, and bake for approximately 1 hour and 30 minutes.
Carefully remove the baking dish from the oven and take off the foil, keeping your hands and face away from the hot steam. Using a heat safe turkey baster, carefully transfer the cooking liquid from the baking dish into a heat-safe bowl or measuring cup. Leave the chicken and vegetables in the baking dish.
In a separate small bowl, whisk the cornstarch and cold water together until completely smooth. Whisk the cornstarch mixture into the hot cooking liquid until well combined. Pour the gravy back over the chicken thighs and vegetables, coating everything evenly.
Increase the oven temperature to 450°F and return the baking dish to the oven, uncovered, for approximately 20 minutes, until the gravy is bubbling and the chicken has a little color on top. Garnish with freshly chopped parsley (optional) and serve warm.