In a small bowl, combine the coriander, cumin, garam masala, 1 tbsp tandoori masala, kashmiri chili powder, and salt. This is your spice mix.
Heat 4 tbsp vegetable oil in a pot over medium heat until it shimmers.
Add the garlic ginger paste and cook until it stops spluttering.
Turn the heat to medium low. Add the spice mix and stir continuously until it starts to smell really good - around 30-40 seconds. Watch it carefully. If you burn the spices at this point you have to start over.
Add the diluted tomato paste and stir to combine. Turn the heat up to medium. Cook for 1 minute.
Add the onion paste and turn the heat up to medium high. Cook, stirring occasionally for about 5-6 minutes. Cover it loosely. It will splatter. If it doesn't your heat is too low. The curry will darken a bit as it cooks.
Turn the heat down to medium low. Add the coconut milk and brown sugar and simmer for another 3-4 minutes.
Add the pre-cooked tandoori chicken and lemon juice. Simmer until the chicken is warmed through - another 2 minutes or so.
Garnish with a bit of cilantro and diced green chili pepper if desired.