Chop the onion and fry in the butter and olive oil until lightly brown.
Add the onion to the slow cooker pot.
Wash potatoes and chop into bitesize pieces then add to slow cooker pot.
Season and flour your chicken breast pieces. Brown lightly in a pan. As I was in a hurry I did not cook through but just seared the outside of each piece.
Add your carrots, mushrooms, mustard, celery salt and herbs.
Add 500ml of chicken stock.
Stir well.
Add 2 bay leaves.
Slow cook on high for 4 hours or low for 8 hours. Can be left longer if desired, including overnight on low.