Add the chicken pieces to a large mixing bowl and coat in the flour.
Heat oil, over high heat, in a wide, heavy-based pan. When hot add chicken thighs and fry for approx 3 mins on both sides. Remove thighs from the pan and add to the slow cooker.
Deglaze the pan by adding the cider vinegar. Add the onions, garlic, carrots, celery and leek and saute, on low heat, for approx 3 mins. Add to the slow cooker.
Add the bell pepper (capsicum), oregano, tomato paste, dijon mustard, chicken stock and potatoes to the slow cooker.
Stir to combine; cover and cook on high for 3-4 hours, or low for 6-7 hours.
Add the mushroom in the last 30 min - 1 hour of cooking.
Stir through the lemon juice and fresh parsley before serving.
Skipping the pre cooking - Pre cooking results in a mellower, more balanced casserole with a slightly sweeter flavour. However, if you are short in time you can skip the pre cooking. Omit the oil and cider vinegar and whisk the flour with the cicken stock before adding to the slow cooker. Thickening the Gravy - If you prefer a thicken gravy then you can thicken by adding a slurry. In the last 30 mins of cooking, remove 1/2 a cup of the broth and mix together with 2tbsp of flour / corn flour. (add more flour depending on how thick you like it) Pour the mixture back into the slow cooker and stir. Cook on high for 30 mins to allow to thicken. Gluten Free Version - To make this dish gluten free you can skip the flouring stage and be sure to use a gluten free chicken stock. Storage - Allow the casserole to cool; store in an airtight container and refrigerate for up to 3 days or freeze for up to 3 months.Nutritional information is a rough guide only, calculated using an online nutrition calculator.