If using cashews, drain and blend them with the onion, tomatoes, and ginger-garlic paste into a smooth paste. Without cashews, blend onion, tomatoes, and ginger-garlic paste alone with 2 tablespoons water.
Sauté the corn
Heat 1 tablespoon oil in a wide pan on medium. Add the corn and sauté 2 to 3 minutes until the kernels look a little dry and lightly toasted. Transfer to a plate.
Cook the gravy
In the same pan, heat 2 tablespoons oil or ghee. Add cumin seeds and let them crackle. Add the slit green chili if using.
Pour in the blended paste. Cook 8 to 10 minutes on medium, stirring often, until the paste thickens, darkens slightly, and no longer smells raw. Splash a little water if it sticks.
Add turmeric, Kashmiri chili powder, coriander powder, and salt. Cook 1 minute until fragrant.
Add 1/2 cup water and bring to a gentle simmer. The gravy should look glossy and spoonable, not watery. Add a little more water only if it is too thick to coat the vegetables.
Add corn and capsicum
Return the sautéed corn to the pan. Simmer 2 minutes.
Add the capsicum squares. Cook on medium 3 to 4 minutes, stirring gently, until the edges soften but the pieces still look bright and hold their shape.
Finish
Turn the heat to low. Stir in cream, butter if using, and crushed kasuri methi. Warm 1 to 2 minutes. Do not boil hard after adding cream.
Taste for salt. Finish with garam masala and cilantro. Add a spoon more cream on top if you like it richer.
Notes
Sauté frozen corn 2 to 3 minutes before adding it to the gravy so the masala stays thick.Add capsicum only after the gravy is ready, and cook 3 to 4 minutes so the peppers stay bright and crisp-tender.Cook the onion-tomato paste until the raw smell goes; a rushed paste tastes sharp.Add cream off a hard boil so the gravy stays smooth.Store leftovers in an airtight container in the fridge up to 3 days. Reheat gently with a splash of hot water.