Rinse and sort the kidney beans, discarding any broken pieces. Place them in a large bowl and cover with plenty of water. Soak overnight, then drain and reserve 3 cups of the soaking liquid (bean water).
In a large pot, heat olive oil over medium-high heat. Add the sausage and cook until browned, about 5-6 minutes. Remove and set aside.
Add onion, celery, and bell pepper to the same pot. Sauté until softened, about 5 minutes. Stir in the garlic and cook for 1 minute more.
Add the soaked beans, Cajun seasoning, bay leaves, 3 cups reserved bean water, and 5 cups fresh water. The liquid should cover the beans by at least 2 inches.
Bring to a rolling boil and cook uncovered for 30 minutes, stirring occasionally. Aftter 30 minutes, add about 2 cups of fresh water to allow the water to cover the beans by 2 inches.
Reduce the heat to low, cover, and simmer for 1-2 hours, stirring occasionally, until the beans are creamy and tender. Add more water as needed to keep them submerged.
Mash some of the beans against the side of the pot to thicken the mixture.
Remove the bay leaves, then stir in the sausage and chopped green onion.
Season with hot sauce to taste and serve over cooked white rice. Garnish with extra green onion if desired.
Notes
The 30-minute boiling step is essential for food safety. Red kidney beans contain phytohaemagglutinin, a toxin that must be destroyed by boiling at high temperatures. If you don't have 30 minutes to boil, I would suggest boiling uncovered for atleast 10 minutes.
This dish tastes even better the next day. Store leftovers in the refrigerator for up to 4 days or freeze for up to 3 months.