In a large saucepan of 2 cups water, cook rice according to package instructions; set aside.
Heat vegetable oil in a large stockpot over medium heat. Cook sausage until lightly browned about 3-4 minutes; set aside
Add onion, bell pepper and celery. Cook, stirring occasionally, until tender, about 3-4 minutes.
Stir in tomato paste, garlic and Cajun seasoning until fragrant, about 1 minute.
Stir in red beans and chicken stock, bay leaf and sausage; season with salt and pepper, to taste.
Bring to a boil; cover, reduce heat and simmer for 15 minutes.
Uncover; continue to simmer until reduced, an additional 15 minutes.
Using a wooden spoon, mash beans until slightly thickened, if desired; season with salt and pepper, to taste.
Serve immediately, topped with rice and garnished with parsley, if desired