Heat the olive oil in a large frying pan and brown the sausages all over. Transfer to the slow cooker.
Then add the onion, carrots, celery and garlic to the slow cooker, followed by the butter beans.
Pour in the chicken stock and add the Dijon mustard, thyme, rosemary, Worcestershire sauce and black pepper. Stir well.
Cook on LOW for 6–7 hours or HIGH for 3–4 hours, until the vegetables are tender and the sausages are cooked through.
About 10–15 minutes before serving, stir in the fromage frais and spinach. Leave the lid on until the spinach has wilted.
For a thicker sauce, gently mash a few butter beans against the side of the slow cooker and stir through.