In a small bowl, combine the mix powder, tandoori masala, Kashmiri chili powder, kasoor methi and salt. This is your spice mix.
Heat 4 tbsp vegetable oil in a pot over medium heat until it shimmers.
Add the green pepper, skin side down. Cook for about 2 minutes. Add the remaining chopped onion and continue to cook until the onion starts to brown on the edges and the green pepper scorches a bit. You really want the colour on the green pepper skin. It's key to the taste of a jalfrezi.
Add the garlic ginger paste and cilantro stems and cook until the garlic ginger paste 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. It will stick a bit. That's OK - you will deglaze with the tomato mixture.
Add the diluted tomato paste and stir to combine. Turn the heat up to medium. Cook for 1 minute, scraping up any spice mix that stuck to the pan..
Add the onion paste and turn the heat up to medium high. Cook, stirring occasionally for about 4 minutes. It will darken a bit as it cooks. Don't worry if it seems a bit dry. The chicken will release juices as it cooks and you should be left with a nice amount of sauce.
Turn the heat down to medium low. Add the chicken and green chilies and stir. Cover and simmer until the chicken is done. It's done when you measure the internal temperature and it says 170F, about 12-15 minutes. An instant read thermometer is a handy thing to have here...
If the curry is a bit thick add a bit of water or chicken stock and stir. Add the cherry tomatoes. Let the chicken jalfrezi simmer another minute or so. Taste for salt and adjust as needed. It may be a bit under salted at this point.
Garnish with a bit of cilantro if desired.