In a large mixing bowl, combine the chicken thighs, salt, ground black pepper, smoked paprika, lemon juice, chopped parsley, and minced garlic. Mix well to ensure the chicken is evenly coated. Let it marinate for at least 30 minutes, or up to 2 hours in the refrigerator for deeper flavor.
Preheat your oven to 425°F (220°C). This high heat will help to crisp up the chicken skin and roast the potatoes perfectly.
While the chicken is marinating, slice the red onion and cut the baby potatoes in half. In a separate bowl, toss the potatoes with olive oil, a pinch of salt, and brown sugar until they are well coated.
In a roasting pan or baking dish, arrange the marinated chicken thighs skin-side up. Scatter the sliced red onion and coated baby potatoes around the chicken.
Place the roasting pan in the preheated oven and roast for about 35-40 minutes, or until the chicken reaches an internal temperature of 165°F (75°C) and the potatoes are tender and golden brown.
Once cooked, remove the pan from the oven. Let it rest for about 5 minutes before garnishing with additional chopped parsley if desired. Serve the Spanish Chicken and Potatoes warm, enjoying each flavorful bite.