Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Melt 1/2 the butter in the pot. Add onion, bell pepper, and celery. Cook until soft, about 5 minutes. Transfer vegetables to a bowl. Add bacon and cook until brown and crisp, about 5 minutes. Remove bacon and drain on a paper towel-lined plate. Add chicken and boudin sausage. Cook in rendered bacon fat until chicken is no longer pink inside and juices run clear, 5 to 10 minutes. Drain and discard bacon grease and turn off Saute function.

Pour in cannellini beans, chicken broth, and cooked onion mixture. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 10 minutes. Allow 10 to 15 minutes for pressure to build.

Meanwhile, melt remaining butter in a small skillet over medium heat. Stir in minced garlic and fresh bread crumbs. Cook until garlic and bread crumbs are browned, 2 to 4 minutes. Transfer bread crumb mixture to a bowl.

Release Instant Pot® pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Remove boudin sausage, cut into 1-inch pieces, and return to the pot. Stir in Cajun seasoning, hot sauce, and cooked bacon. Ladle cassoulet into serving bowls and top each serving with toasted bread crumbs.