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.