Turkey Balti

I used to cook a curry dish for our International Day at school.
It became so popular I was asked to do it every year, making a huge pot
so that the staff could polish off the remainder!
Here is the "domestic" quantity.

2lb diced turkey
3 tablespoons oil
3oz finely chopped onion
2 cloves finely chopped garlic
2 tins chopped tomatoes
1 teaspoon ground turmeric
½ teaspoon red chilli powder (or to taste)
2 teaspoons salt
2oz butter or ghee
1 tablespoon ground cumin
1 tablespoon ground coriander
1” fresh root ginger, grated
1 teaspoon garam masala

Put the oil in a deep frying pan and stir fry the onion and garlic for two minutes over a high heat.
Add the tomatoes and cook uncovered on medium heat for 10 minutes or until liquid is reduced.
Add the turkey pieces, turmeric, red chilli powder and salt. Stir well and cook, covered, on low heat for 25-30 minutes. If necessary, turn heat up to reduce liquid for a few minutes – it should be a thick gravy.
Add the butter or ghee, ground cumin, ground coriander, and grated ginger and simmer for 10-15 minutes. Stir in the garam masala and cook for a further 10 minutes.