Taste your tandoori masala. If it tastes like salt you have a salty product. Leave out the salt specified in this recipe. You can adjust at the end.
In a small bowl, combine the coriander, cumin, garam masala, 2 tsp tandoori masala, kashmiri chili powder and kosher salt if you are using it. 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. You'll understand what I mean when you do it.
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 2-3 minutes.
Add the heavy cream. Stir to combine.
Taste for salt. If you've left the salt out it's time to decide if it needs more. I can't help you here. Not until I take over global production of tandoori masala anyway.
Grab a spoonful of the sauce. Look at it. If it seems a little thick add a bit of water. Maybe 2 or 3 tablespoons. Stir and warm it through. Your goal is the lush restaurant gravy texture you know and love.
Garnish with a bit of cilantro or fried shallots if desired. Drizzle a little more cream overtop if you want "the look". Enjoy!