Dice the chicken into roughly bite-sized pieces and slice the sausage. Dice the onions, peppers and celery, all relatively small. Finley chop or crush the garlic.
Warm the oil in a wide, heavy based dish over a medium-high heat then add the chicken and brown all over. Once cooked through, remove from the pan and set aside. (If you prefer, you can briefly cook the shrimp now too and set those aside, but it also works to add at the end and cook in the rice and last of the liquid). Add the Andouille sausage and brown lightly all over, a couple minutes.
Once the sausage is gently browned, add the onion, pepper and celery. Reduce the heat and/or add a little extra oil as needed to soften but take care they don't burn. Cook, stirring now and then, for a couple minutes then add the garlic. Cook until all of the vegetables are softened, around 5 minutes or so.
Add the thyme, oregano, paprika, cayenne and tomato paste. Cook for a minute or two then add the crushed tomatoes and stock. Bring to a simmer then add the rice and bay leaf. Reduce the heat so that it continues to simmer but doesn't boil too strongly, and leave approximately 20 minutes until the rice is just cooked, stirring now and then. The rice may take a bit longer to cook, depending on the variety used, pan etc - you may find it helps to cover part of the time to help speed things up and cook evenly.
Add the shrimp , if not cooked before, when the rice is almost cooked, pressing them under the rice mixture to help them cook through, and add back the chicken (and shrimp if pre-cooked). Turn them over if needed and once the shrimp are cooked through, serve.