Start by gathering your slow cooker and setting it on a stable counter away from overhead cabinets. Pour in the chicken broth, undrained diced tomatoes, tomato sauce, and the uncooked arborio rice directly into the stoneware insert. Add the chopped onion, parsley, olive oil, garlic powder, thyme, pepper, salt, cayenne, and the bay leaf, then stir everything gently until the spices are well distributed throughout the liquid mixture.
Place the lid securely on the slow cooker to trap all the heat and moisture inside. Set the temperature to High and let it cook undisturbed for about 1 hour and 45 minutes. It is crucial not to lift the lid during this time, as slow cookers lose significant heat when opened, which could result in crunchy, undercooked rice grains.
Once the initial cooking time is up, quickly lift the lid and stir in the raw shrimp and sliced Andouille sausage. If you are using the Parmesan cheese, you can add it now or wait until the very end, but adding the meats is priority. Give it a good stir to ensure the shrimp are submerged in the hot rice mixture so they can poach gently.
Cover the pot again and continue cooking on High for another 10 to 15 minutes. You are looking for the shrimp to turn opaque and pink, and for the rice to be tender but still have a slight "bite" to it (al dente). Be careful not to overcook at this stage, or the shrimp can become tough and chewy.
Turn off the slow cooker and remove the stoneware from the heating element if possible to stop the cooking process. Locate and remove the bay leaf, as it has done its job and is not meant to be eaten. Give the risotto a final stir, garnish with green onions if desired, and serve immediately while it is hot and creamy.