For the roux, heat a small pan over a medium heat and melt the butter.
Add the flour and stir well to create a paste. Allow it to cook for two minutes, then pour into a bowl.
For the filling, melt the butter in a lidded casserole, then add the onions and season with salt and freshly ground black pepper. Cover with the lid and reduce the heat to sweat the onions for five minutes, or until softened.
Turn up the heat and add the ground coriander, cumin and the pork. Stir for a few minutes until the pork is golden-brown all over.
Add the stock, cover with the lid and transfer to the oven to cook for 45-60 minutes, until the pork is tender.
While the pork is cooking, heat the olive oil in a pan over a high heat. Add the mushrooms and fry for 2-3 minutes, until they are golden-brown.
After the pork has been cooking for 30 minutes, add the fried mushrooms to the casserole.
When the pork is cooked, remove the pork and mushrooms from the cooking liquid with a slotted spoon and place into a dish. Set aside in a warm place.
Add the cream to the liquid in the pot and bring to the boil over a medium heat. Simmer for 3-4 minutes then, to thicken the sauce, gradually add 2-3 tablespoons of the roux, whisking all the while, as the sauce boils.
Add the chopped parsley and return the pork and mushrooms to the casserole. Season, to taste, with salt and freshly ground black pepper.
Either spoon equal portions of the casserole into individual ovenproof dishes, or transfer all of the casserole into a large pie dish.
Turn the oven up to 180C/350F/Gas 4.
For the mashed potatoes, half-fill a large pan with cold water and add the potatoes and a pinch of salt. Bring the water up the boil and cook the potatoes for ten minutes.
Drain off all but about 4cm/1½in depth of the water. Return the potatoes to a very low heat and continue to simmer. (Don't be tempted to stick a knife into them - the skins will break and they will just break up and become soggy if you do.)
About 18-20 minutes later (or when cooked through), drain and allow the potatoes to cool slightly. Peel the potatoes while they are still hot, holding them in a tea towel to prevent scalding your hands.
Return the peeled potatoes to the pan and mash well. Add the butter and continue mashing until the mixture is free from lumps.
Add the boiling milk (or milk and cream mixture) if required, to achieve a pleasing texture of mashed potato, then season, to taste, with salt and freshly ground black pepper.
To make the pie(s), arrange the mashed potato over the top of the filling in the pie dish (or individual heatproof bowls if using) and lightly score the surface with a knife to create a diamond pattern.
Place the pie dish(es) onto a baking tray and transfer to the oven to cook for 30-40 minutes, or until golden-brown on top and the filling is hot.
Meanwhile, for the beans, place the beans into a pot of salted boiling water and boil for 3-4 minutes, or until they are just tender. Drain immediately and rinse under cold running water to stop them cooking.
Heat the oil in a large frying pan over a medium heat. When hot, add the mustard seeds. As soon as the mustard seeds begin to pop, add the garlic and cook until golden-brown.
Add the crushed red chilli and stir for a few seconds, then add the green beans, salt and sugar and stir well.
Turn the heat down to low and cook the beans for 7-8 minutes, then season, to taste, with salt and freshly ground black pepper.
To serve, serve generous portions of pie onto plates and place a spoonful of green beans alongside.