Boil the potatoes in water for approximately 15 minutes.
Place a wok over a hot flame and pour in the oil
Add the carrots, celery, garlic, onion, parsnips, and spinach to the wok and stir-fry for approximately 10 minutes. You want the vegetables starting to cook but still crunchy. You don't want them soft at this point.
add the stock, peas, and mustard to the pan and mix well. Add the salt and pepper to season.
Drain the potatoes, return them to the saucepan and mash.
Place the vegetable mix into a baking dish, then spoon the mash over the top.
using a fork fluff up the mash to give it some texture.
Place into the oven and bake for approximately 10-15 minutes.
You want the top to be golden brown and not to burn.