Pre-heat the oven to 400°F with the rack in the middle
Rub chicken pieces down with olive oil, separating the skin from the meat but leave the skin intact
Apply the spice mix, rubbing under the skin and on top of the pieces of chicken
Add half a cup neutral oil to a hot skillet. Add the onion and cook until it begins to soften and turn transclucent, then add celery, bell pepper, and garlic. Add 1/4tsp salt and stir to combine. Cook on medium-low heat (watching carefully so it doesn't burn) until all the vegetables are very soft or broken down
Transfer the oil and aromatics to your baking dish and add 3 cups rice, stirring to get them well combined. Add oil if needed to ensure the rice is coated and glossy, then spread the rice out flat
If using shrimp, layer them on the rice, gently pressing them into the rice layer. Sprinkle a bit of cajun spice on top of each shrimp
Arrange the chicken pieces on top of the rice and shrimp, leaving space between each piece of chicken.
Pour the stock gently around the chicken into the rice
Cover the dish pan tightly with aluminum foil and place it in the oven. Roast for 40 minutes and then remove the foil
Continue roasting for an additional 30-40 minutes until the chicken is cooked through and the rice has soaked up all of the liquids
If the chicken isn't super brown and you're feeling fancy, hit it with the broiler for another 5 minutes at the end
Garnish with some chopped celery leaves or parsley, etc, if desired