Preheat the oven to 425°F. Lightly grease a large rimmed baking sheet or roasting pan.
Place the potatoes, carrots, bell pepper, and onion on the prepared pan. Drizzle with 2 tablespoons of olive oil and toss to coat.
In a small bowl, combine the smoked paprika, Italian seasoning, garlic powder, onion powder, salt, and black pepper.
Pat the chicken thighs dry with paper towels. Rub them with the remaining tablespoon of olive oil, minced garlic, and about half of the seasoning mixture.
Sprinkle the remaining seasoning over the vegetables and toss until evenly coated.
Arrange the chicken among the vegetables, leaving a little space between the pieces so everything can roast evenly.
Roast for 30–35 minutes, stirring the vegetables once around the 20-minute mark. Continue cooking until the vegetables are tender and the chicken reaches an internal temperature of at least 165°F.
Drizzle the chicken and vegetables with fresh lemon juice. Let the chicken rest for about 5 minutes, then sprinkle with fresh parsley and serve warm.