Combine Indian restaurant spice mix, kasoor methi, chili powder and salt in a small bowl.
Dilute the tomato paste with enough water to get to the consistency of passata.
Heat your frying pan (don't use non-stick) briefly over medium heat.
Add 2 Tbsp vegetable oil. Use all the oil specified. It's important.
When the oil starts to shimmer in the garlic ginger paste and the green chilies. Cook it, stirring constantly, until it stops sputtering.
Turn down the heat and add the spice mix. This is the critical step. Stir it constantly for 30 seconds. If it starts to darken lift the pan off the heat. You want the spice mix to cook in the oil but not burn.
Turn the heat up to medium high. Add the diluted tomato paste and stir until bubbles form (the oil will likely separate). This takes around 30 seconds to one minute depending on the heat.
Add 3 oz of curry base. Stir until bubbles form (little craters really), around 30 seconds. Watch the edges of the pan. The curry can stick here.
Now add 6 oz of curry base and stir briefly. Let it cook until the bubbles form again. This takes 1-2 minutes.
Add the rest of the curry base and let cook until the bubbles form. Turn the heat down to low and add the pre-cooked keema.
Let the curry simmer for about 3 minutes. If it gets too thick add a bit more curry base. Don't add water.
Add the cooked potatoes and cook another 2-3 minutes to warm them through.
Stir in the lemon juice, taste and adjust salt and lemon to taste.
Garnish with thinly sliced green onions and green chilies if you like.