Preheat oven to 425°. In a large bowl, combine potatoes, carrots, onion, oil, garlic, 1 teaspoon thyme, 3/4 teaspoon salt and 1/2 teaspoon pepper; toss to coat. Transfer to a 15x10x1-in. baking pan coated with cooking spray.
In a small bowl, mix paprika and the remaining thyme, salt and pepper. Sprinkle chicken with paprika mixture; arrange over vegetables. Top with lemon slices. Roast until a thermometer inserted in chicken reads 170°-175° and vegetables are just tender, 35-40 minutes.
Remove chicken to a serving platter; keep warm. Top vegetables with spinach. Roast until vegetables are tender and spinach is wilted, 8-10 minutes longer. Stir vegetables to combine; serve with chicken.