Heat your oven to 200ºC/Fan 180ºC/Gas 6. Thinly slice the celery. Trim the spring onions. Slice them. Peel the garlic and chop roughly.
Crumble the chicken stock cube into a measuring jug. Pour in 300ml boiling water. Stir to mix. Bring a pan of water to the boil.
Cut the potatoes into large chunks (no need to peel them) or simply halve if they’re quite small. Boil for 15–20 mins till tender. Drain well. Return to the pan. Press each piece of potato with the back of the spoon to crush lightly.
While the potatoes boil, toss the chicken in a bowl with 2 tsp olive oil and some salt and pepper. Heat a large frying pan. Add the chicken and fry for 2 mins on each side. Cook and stir for 1 more min till the chicken is golden. Set aside on a plate.
Turn the heat down under the pan. Add half the butter and all the celery. Cook for 2 mins. Add the spring onions and garlic. Cook for 3 mins. Thickly slice the mushrooms. Add to the pan. Increase the heat slightly. Cook and stir for 2 mins.
Turn the heat right down and add the flour. Gradually stir in the stock to make a sauce. Simmer for 2-3 mins till any raw flour taste has been cooked out. Turn off the heat and stir in the chicken. Tip the mix into a 2-person pie dish.
Dot the top of the pie with the remaining butter. Put the pie in the oven for 20 mins till golden on top. Serve straight away.