Preheat the oven to 425°F. In a large bowl, add the potatoes, onion, oil, garlic, ¾ teaspoon salt, 2 teaspoons fresh rosemary (or ½ teaspoon dried rosemary), and ½ teaspoon pepper; toss to coat. Transfer the vegetables to a 15x10x1-inch baking pan coated with cooking spray.
In a small bowl, mix the paprika with the remaining salt, rosemary and pepper. Sprinkle the chicken with the paprika mixture and arrange it over the potatoes and onions.
Roast until a thermometer inserted in the chicken reads 170° to 175° and the vegetables are just tender, 35 to 40 minutes. Remove the chicken to a serving platter, but keep it warm. Editor's Tip: If the potatoes get too brown before the chicken cooks, move them around with a spatula so they cook evenly on all sides. If the chicken happens to cook before the vegetables, remove it and allow the potatoes to cook until just tender.
Top the vegetables with spinach. Roast until the vegetables are fully tender and the spinach wilts, 8 to 10 minutes longer. Stir the vegetables to combine; serve with the chicken. If desired, serve with additional fresh rosemary and lemon wedges.
