Heat the oil in a large frying pan over a medium heat and fry the onion, celery and carrots for 5 minutes until they begin to soften. Add the garlic and mushrooms and fry for 3 minutes more.
Add the chicken and oregano to the pan and continue frying for another ten minutes until the chicken has cooked through and is beginning to brown. Cut the apple into 8 wedges, add to the pan and fry for 2 minutes more.
Add the mustard and stir in well, then pour in the cider and increase the heat. Cook on a high heat for 3-5 minutes until the alcohol has burnt off and the liquid has reduced by about a quarter.
Turn off the heat, stir in the creme fraiche and season to taste with salt and pepper. Transfer to a large baking dish.
Make the crumble topping. Put the flour, mustard powder and some salt and pepper in a bowl and mix well. Add the butter and rub in until you have a breadcrumb like mixture. Stir in the cheese and hazelnuts and mix well.
Top the chicken mix with the crumble and bake in the oven for 25 minutes until the crumble is golden brown and crunchy. Serve with vegetables.