Place the chicken in a bowl with 6 tablespoons of the flour plus 1/2 tsp each of salt and pepper. Toss to cover the chicken in the flour and seasoning.
Heat the oil over a high heat in a large frying pan (skillet) and add the chicken. Brown all over (it doesn't need to be cooked through at this point). Remove from the pan with a slotted spoon and put to one side.
Place the butter in the same frying pan and melt over a low-medium heat. Add the onions, garlic, thyme and celery salt and cook for 5 minutes until the onion softens. Sprinkle on the remaining 3 tbsp of flour and stir for a minute (it will be lumpy).
Pour in a splash of the stock and stir, using a whisk until combined. Continue to add in stock, a little at a time, whilst stirring, until all the stock is added and you have a smooth sauce with no lumps (the onions will still be in there though - so it will look a little lumpy because of that).
Add the milk and continue to stir over the heat until the sauce thickens, then add in the lemon juice.
Add the mushrooms, the chicken and the remaining 1/2 tsp of salt and pepper. Place a lid on the pan and simmer gently on the hob for 20 minutes.
**Alternatively you can transfer to a casserole dish at this point. Cover with foil and place in the oven at 175C/350F for 30 minutes**
Remove the lid and stir in the cream, then heat through for a further 5 minutes (place back in the oven if oven cooking).
Remove the lid and test for seasoning. Add a little more salt and pepper if needed.
If you'd like the sauce to be any thicker, then at this point you can stir in the cornflour slurry. Add a little splash at a time, whilst stirring, until you get the thickness you want.
Serve the casserole with vegetables (I like to serve mine with sprouts, peas and sweetcorn) and a sprinkling of parsley.
You can make this dish ahead, then cool, cover and refrigerate for a day. Heat through thoroughly in the oven (covered) at 175C/350F for approx 20-25 minutes. Or heat through on the hob for 10-15 minutes on a medium heat until the chicken is hot all the way through. Add a splash of cream or milk if needed to loosen it up..
Make the casserole, then cool, cover and freeze. Defrost thoroughly in the refrigerator and heat through thoroughly in the oven (covered) at 175C/350F for approx 20-25 minutes. Or heat through on the hob for 10-15 minutes on a medium heat until the chicken is hot all the way through..
Reducing the quantities:
You can halve the quantities if necessary - no change to the cooking times. .
Can I make it in the slow cooker?
I haven't tried this one in the slow cooker yet. If you try it, please let me know how it went. I'll update the notes when I test it using the slow cooker..Nutritional Information is per Serving WITHOUT Serving suggestion of Mash Potato, Sprouts, Peas and Sweetcorn.