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.