Boil potatoes in salted water with a drizzle of oil till tender.
Meanwhile, heat 4 tablespoons of olive oil in a large frying pan. When hot add onion and fry on high for 5 minutes. Add salt, fry, lower heat and sauté until rich golden brown.
Raise heat slightly and add garlic and green chillies and fry until softened.
Add mince and cook and stir until browned. Add all herbs and seasonings and cook for an additional 4 to 5 minutes, then add the mushrooms and carrots and cook till softened.
Add tomato passata and puree; stir well and cook until oil separates.
Add boiling water until covered. Bring to boil then lower heat and simmer for 40 minutes.
Once potatoes are tender, drain well and put through potato ricer. Add milk, butter and salt (2 teaspoons, or to taste). Stir well. Add half of the cheese and stir till smooth and mashed.
Remove mince from heat and let cool slightly, then transfer to deep baking dish. Ladle spoonfuls of mash on top of the mince. (I prefer to start around the inside edge of the baking dish, then I add potatoes in the middle and smooth over.)
Top with the remaining cheese and sprinkle some herbs and seasonings.
Bake in a pre-heated 200 C / Gas 6 oven for 45 minutes or until mince is piping hot and a the top is golden brown.
I sometimes use a Knorr® lamb stock cube dissolved in boiling water instead of just plain boiling water.