Heat oil in a wide based pan and add the diced onion and sauté for 5 minutes before adding the garlic. Stir and continue to cook for about 10 minutes until the onions are golden brown in colour.
Once browned reduce the heat and add the tomatoes, ginger, salt, turmeric, cumin and chili (optional). Stir together and cook to reduce until the tomatoes break down into the sauce (about 5 minutes) till the mix turn into an aromatic paste.
Turn the heat up and add the mushrooms and lamb and cook for a few minutes before adding the mince. Stir to coat the mince with the sauce. Once coated reduce the heat and leave it to cook for about 15 minutes. Stir in the frozen peas and leave to cook for a further 5 minutes.
Remove from the heat and spoon into a casserole dish and leave to cool a little.
Pre-heat your oven to 180ºC.
Cover the potatoes with cold water and add a little salt. Bring this to the boil then reduce the heat and leave the potatoes to cook until soft. Once cooked drain and place back in the pan.
Add a little salt, butter and pour in a little milk and mash the potatoes to create a firm mash.
Sprinkle in the garam masala.
Place this mix on top of the mixture and place in the oven for about 25-30 minutes until it’s bubbling.
Sprinkle with fresh green coriander and chopped spring onions to serve.