Preheat oven to 400 degrees F.
On a large baking sheet, toss together the onion, bell peppers, celery, garlic, jalapeƱos, tomatoes, 2 tablespoons olive oil, salt pepper until evenly combined. Give the pan a shake to ensure that everything is on a single layer.
In a medium bowl, toss together the chicken with 1 tablespoon Cajun seasoning. Evenly distribute the chicken onto the baking sheet, among the vegetables, along with the andouille sausage. Roast until the chicken is cooked through and the veggies are beginning to charr, about 15 to 20 minutes.
In a medium bowl, toss the shrimp with the remaining 1 tablespoon Cajun seasoning. Remove the baking sheet from the oven and distribute the shrimp on top in an even layer. Return to the oven and cook until the shrimp is no longer pink, about 5 to 8 minutes. It'll go by fast because the oven is so hot and shrimp takes no time at all. Remove from the oven once more and top evenly with the cooked rice. Return to the oven for another 5 minutes to warm the rice.
Garnish with fresh lemon slices, scallions and parsley before serving. Enjoy!