2 Tbsp. vegetable oil
1 large onion, coarsely chopped
1 medium green bell pepper, coarsely chopped
2 stalks celery, coarsely chopped
1 lb. dried kidney beans, soaked overnight, drained
3 garlic cloves, finely chopped
2 fresh bay leaves
8 cups (or more) low-sodium chicken broth or 4 cups low-sodium chicken broth plus 4 cups water
¼ cup finely chopped parsley, plus more for serving
1 Tbsp. Cajun seasoning
3½ tsp. Diamond Crystal or 2¼ tsp. Morton kosher salt, divided, plus more
1 tsp. ground sage
2 tsp. freshly ground pepper
6 oz. andouille sausage, cut into ½" pieces
2 cups long-grain rice
Heat 2 Tbsp. vegetable oil in a Dutch oven or other heavy, large pot over medium. Cook 1 large onion, coarsely chopped, 1 medium green bell pepper, coarsely chopped, and 2 stalks celery, coarsely chopped, stirring occasionally, until onion is translucent, about 5 minutes. Add 1 lb. dried kidney beans, soaked overnight, drained, 3 garlic cloves, finely chopped, and 2 fresh bay leaves. Cook, stirring often, 2 minutes.
Pour in 8 cups low-sodium chicken broth or 4 cups low-sodium chicken broth plus 4 cups water and bring to a boil. Stir in ¼ cup finely chopped parsley, 1 Tbsp. Cajun seasoning, 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt, 1 tsp. ground sage, and 2 tsp. freshly ground pepper. Reduce heat to medium-low and simmer, adding more broth or water (up to ½ cup) as needed to keep beans submerged, until beans are tender but still intact, 1½–2 hours. Add 6 oz. andouille sausage, cut into ½" pieces, and simmer 30 minutes. Taste and add more salt if needed; remove and discard bay leaves.
Bring 4 cups water to a boil in a medium saucepan. Add 2 cups long-grain rice and remaining 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt and return to a boil. Reduce heat to low, cover, and cook rice until grains are tender and liquid is absorbed, 30–35 minutes.
Divide rice among bowls and ladle beans over. Top with parsley.