Preheat oven to 350°F/180°C.: The moment you set the oven to 350°F/180°C you should notice the warm dry air begin to circulate as it approaches temperature, which primes the dish for even heat. Proper preheating ensures the chicken starts roasting immediately rather than slowly warming, which improves browning and texture. If you skip preheating you may end up extending the cooking time and getting uneven results, so wait until the oven signals readiness.
To a large mixing bowl, whisk the oil and seasonings, then add the chicken thighs and mix so that the chicken thighs are well coated. Marinate for 15-30 minutes if the time allows.: When you whisk the olive oil and spices you will smell the paprika and herbs bloom, a fragrant sign the seasoning is active. Coating the chicken lets the flavors adhere to the surface and begin to penetrate the meat. Marinating even briefly tenderizes the outside and deepens flavor, but avoid marinating too long at room temperature for safety. A common misstep is not fully coating the thighs which leads to uneven seasoning across pieces.
To a baking dish, add onion, garlic, olive oil, butter, hot vegetable stock, hot water, rice, salt, and pepper. And give everything a quick mix.: As you assemble the base, the heat of the stock will release steam and scent, and the melting butter will create a glossy surface on the rice . Stirring ensures the diced onion and minced garlic distribute evenly so every grain absorbs flavor. Be careful to use hot liquids, because cold liquid can interrupt the rice cooking cycle and lead to gummy or unevenly cooked grains.
Place the chicken over the rice.: Nestling the seasoned chicken into the rice allows its juices to baste the grains as it cooks, infusing the rice with savory depth. Visually you want the thighs spaced so air can circulate, not crowded, which helps the edges brown better when uncovered later. A mistake here is overcrowding the dish which can prevent proper browning and create steam pockets.
Cover with foil and bake covered for 30 minutes.: Covering traps steam so the rice absorbs the hot liquids and the chicken cooks gently. During this phase you should hear a faint simmer and smell the blended aromatics rising, and the rice will start to swell as it hydrates. If the foil is not sealed well you may lose steam and end up with undercooked rice, so ensure a snug cover.
Uncover, spray the chicken with oil and bake for 30 more minutes. Broil for a few minutes to give the chicken golden and crisp edges and crust.: Once uncovered you will see the surface dry a bit, and spraying with oil helps the chicken develop a golden crust as it roasts. The exposed heat will crisp edges and concentrate flavors, producing audible sizzling and a deeper roasted aroma. Broiling for a short time is optional but creates a desirable color and texture. Watch closely during broiling to avoid burning, as the high heat works very quickly.
Remove from the oven, and allow the chicken and rice to rest for 10 minutes. Fluff the rice (remove the chicken if needed then put it back in), garnish with fresh herbs and serve.: Resting is when the juices redistribute and the steam finishes cooking the grains, so the rice becomes fluffy rather than sticky. When you fluff the rice you will notice separated grains and a fragrant lift of the seasonings. A common oversight is serving immediately, which can leave the rice dense and the chicken juices running out; allow the full rest time for best texture.