Heat the olive oil in a frying pan over medium heat. Add the sausages and brown them for approximately 5–8 minutes, turning regularly so they develop color on several sides. They don’t need to be fully cooked at this stage because they will finish cooking in the slow cooker. Transfer them to a plate and carefully discard excess fat if necessary.

Add the chopped onion, carrots, and optional celery to the same pan. Cook for about 4–5 minutes until the onion begins to soften, then stir in the minced garlic and cook for another 30 seconds. This quick step helps develop flavor before the vegetables go into the slow cooker. Transfer everything to the slow cooker bowl.

Add the chopped tomatoes, drained butter beans, stock, tomato paste, Worcestershire sauce, thyme, mixed herbs, bay leaf, and optional smoked paprika. Stir thoroughly to combine the ingredients and distribute the seasoning evenly. The mixture should look saucy but not excessively watery, as the slow cooker retains moisture during cooking.

Nestle the browned sausages into the tomato and vegetable mixture, making sure they are mostly surrounded by the sauce. Cover with the lid and cook on low for 6–8 hours or high for 3–4 hours, following your slow cooker’s instructions. Avoid lifting the lid frequently because heat escapes and can extend the cooking time.

When the cooking time is nearly complete, check that the sausages are thoroughly cooked and the carrots are tender. Stir gently and taste the sauce, adding black pepper or a little salt if needed. If the casserole is thinner than you prefer, stir in the cornstarch slurry and cook on high for another 10–15 minutes, following your appliance’s guidance.

Remove the bay leaf and sprinkle the casserole with chopped fresh parsley. Allow it to stand for a few minutes before serving so the sauce settles slightly. Spoon the sausages, beans, vegetables, and rich tomato gravy into warm bowls. Serve your Slow Cooker Sausage Casserole Recipe with mashed potatoes, crusty bread, rice, or your favorite vegetables.