Preheat the oven to 400 degrees.
Place peppers, onions, celery, chicken, and sausage on a large baking sheet or sheet pan with sides that are 1-2 inches tall.
In a medium bowl, combine melted butter and Cajun seasoning. Pour half of the seasoned butter over ingredients on the sheet pan. Place shrimp into the bowl with the remaining seasoned butter. Set aside until needed.
Place the uncovered sheet pan in a preheated oven to roast for 20 minutes or until vegetables have some brown edges and chicken is almost cooked through.
Remove from the oven and add the rice and tomatoes. Toss with a spoon to combine, then flatten and spread out over the bottom of the sheet pan. Place shrimp with seasoned butter on top. Put the sheet pan back into the oven for 7-8 minutes or until shrimp are opaque and cooked through.
Remove the sheet pan and stir with a spoon to combine all the ingredients. Sprinkle the top with diced green onions before serving.