Wash mince well and drain in a fine strainer. When using potatoes, I normally microwave them for 1-2 mins covered in the microwave before adding them to the dish.
Heat a karhai or wok over med-high; add 2 tbsp oil. Add onions and stir around. Add the black peppercorns and cloves, and about a 1/3 of the ginger. Stir this occasionally for 3-4 mins until beginning to colour.
Add ginger-garlic paste and fry for 30 seconds. Add the potatoes and stir over med-high for 1 and a half minutes Add tomatoes and turmeric; stir for 1 minute Add a 1/2 cup of hot water and cover over medium heat for 6 mins, shaking occasionally while covered to make sure the mixture doesn't stick.
Uncover and stir. Add another 1/4 cup of water and cook further 5 mins covered.
Add chilli powder and cumin powder. Mix over med-high, stirring often for 2 minutes Add 1/4 cup of water and stir. Let the mixture cook for 1 minute, then add the keema. Stir constantly over med-high for 1 minute, breaking up any lumps of mince.
Add garam masala and 1 tsp salt. Stir the mixture slowly and regularly for 2 minutes Add about 1/3 of remaining ginger and stir through. Add the chopped green chilli and stir through. Cook the keema covered for 10 mins over medium heat, until the mixture is drying and the potatoes are tender.
Uncover and add sugar, stirring slowly for 1 minute. Remove 1 of the potatoes onto a small plate, mash with a fork and return to the mixture. Mix it through.
Add half the remaining ginger and the whole mint leaves. Increase heat to med-high and stir regularly for 3-5 mins until you have a dry keema.
Place in a serving dish and garnish with remaining ginger, some chopped mint, and wedges of tomato and lemon.