After a bit of time, about 5-7 minutes, when the beans are good and soft, mashed them up a bit. This will give a nice thickness to the stew, but don't over do it - you still want some whole beans in there. You can always add a bit of water or additional stock to get the consistency you want. Add the mustard and give it a good stir. When its all looking good, get the sausages back in there and give them a few minutes to warm up again and mingle with the other flavors.