Heat the butter and oil in a large frying pan, cook the onion for 8-10 mins until softened and starting to caramelise. Meanwhile put the flour and a little salt and pepper in a bowl and toss the chicken in it. Add the garlic and chicken to the pan and cook for 4-5 mins more until the chicken is starting to brown.
Transfer to your slow cooker, along with the rest of the ingredients, including the soaking water from the porcini mushrooms. Give it a good stir. Cook on Low for 7 hours or High for 4 hours. Remove the bay leaves and serve with Dijon mustard on the side.