1. Heat your oven to 200°C/Fan 180°C/Gas 6. Put a large pan of salted water on to boil. Thinly slice the celery, discarding the dry ends. Trim the roots and ragged tops off the spring onions. Thinly slice the spring onions (green and white parts). Peel the garlic cloves. Crush or finely chop them.
2. Scrub the potatoes and quarter them. The water should be boiling by now, so add the potatoes and simmer them for 20 mins till tender.
3. While the potatoes are cooking, warm a deep frying pan for 1 min, then add 2 tsp olive oil and the chicken. Fry for 4-5 mins, turning once, till golden brown. Lift the chicken out of the pan and set aside on a plate.
4. Turn the heat down under the pan. Add 2 tsp more oil and the celery, spring onions and garlic. Tear the mushrooms into the pan and fry for 5 mins, till the veg look juicy.
5. Stir in 1 tbsp flour and cook, stirring, for 2 mins. Add 100ml milk and cook, stirring, till the milk and veg are smoothly combined. Repeat, adding the milk in 100ml splashes till you’ve used up 400ml. Add the chicken back to the pan. Bring to the boil, then turn the heat down and simmer for 5 mins till the sauce has thickened a little.
6. While the sauce simmers, coarsely grate the cheddar. The potatoes should be cooked by now. Drain them well, then tip back into the pan. Roughly crush them with a masher or fork then stir in 1 tsp olive oil and a pinch of salt and pepper.
7. Stir the cheddar and 1 tsp mustard into the sauce. Taste and add a pinch more salt and pepper if you think it needs it.
8. If your frying pan is ovenproof, spoon the potato over the top and slide the frying pan into the oven. Otherwise, spoon the pie filling into an ovenproof dish, cover with the crushed potatoes and bake in the oven for 25 mins till golden and bubbling.
9. While the pie bakes, roughly chop the parsley. Serve the cheesy chicken and mushroom pie garnished with the chopped parsley.