Sort the great northern beans, removing any broken beans, stones, and dirt clods. Put the beans in a strainer and rinse under running water.
In an Instant Pot or other pressure cooker, heat a tablespoon of oil over Sauté mode (medium heat) until the oil is shimmering. Add the onion, green pepper, celery, and garlic, and sprinkle with ½ teaspoon of fine sea salt. Sauté, stirring occasionally with a flat edged wooden spoon, until the onions soften, about 5 minutes. Stir in the Cajun seasoning (and 1 teaspoon of fine sea salt if using homemade or salt-free Cajun seasoning), and cook for 1 minute to toast the spices.
Stir the rinsed beans and smoked sausage into the pot. Pour in the 6 cups of water, then stir in ½ teaspoon of baking soda.
Lock the lid on the pressure cooker. Cook on high pressure for 35 minutes in an Instant Pot or other electric pressure cooker, or for 35 minutes in a stovetop PC. (In an Instant Pot, use “Manual” or “Pressure Cook” mode set for 35 minutes). Let the pressure come down naturally; if you’re in a hurry, you can quick release any remaining pressure after 15 minutes of natural release.
To thicken the bean broth, set the pressure cooker to Sauté mode, bring the pot to a simmer, and simmer, uncovered, for 15 minutes.
Ladle the beans into bowls, serve with hot sauce, and enjoy!
