Preheat the oven to 220ºC, fan 200ºC, gas 7. Heat the oil in a large pan, add the onion and cook for 5 minutes. Add the garlic, lamb, soy sauce, mustard, stock and peas. Stir, then bring to the boil and cook for 8 minutes. Mix the cornflour with 2 tablespoons of water, add to the pan and cook for 1 minute.
Meanwhile, put the potatoes, parsnips and milk in another pan and heat gently, covered, for 5 minutes. Crush roughly with a potato masher.
Spoon the lamb filling into a 1.25-litre pie dish and scatter over the mint. Top with the potato mixture, scatter over the Cheddar and bake for 10-15 minutes, until the top is golden.