Preheat oven to 425°. Place potatoes, onion and sliced lemon in a large bowl; toss with 2 tablespoons oil, garlic and 1/2 teaspoon each salt, rosemary and pepper. Spread evenly in a greased roasting pan. Roast on an upper oven rack 20 minutes., Meanwhile, toss chicken with paprika and the remaining salt, rosemary and pepper. In a large skillet, heat remaining oil over medium-high heat. Brown chicken in batches., Place chicken over potato mixture. Roast until a thermometer inserted in chicken reads 170° and potatoes are tender, 15-20 minutes. Remove chicken from pan. Immediately add spinach to vegetables, stirring to wilt slightly. Serve with chicken and, if desired, lemon wedges.