Place the cashews in a small bowl and cover them with warm water. Set aside while you prepare the remaining ingredients.
Heat 1 tablespoon butter in a pan over low heat. Add the turmeric powder, Kashmiri red chilli powder and salt. Stir for about 30 seconds.
Add the paneer cubes and cook for 2–3 minutes, turning occasionally, until lightly golden around the edges. Remove the paneer from the pan and keep aside.
In the same pan, heat the mustard oil. Add the tomatoes, onion, ginger, garlic and green chillies.
Cook over low to medium heat, stirring occasionally, until the onion and tomatoes have softened and are completely cooked.
Transfer the cooked vegetables to a blender. Add the soaked and drained cashews along with a small splash of water. Blend until completely smooth.
Add 1 tablespoon butter to the same pan. Once melted, pour in the blended tomato-onion mixture.
Add the Kashmiri red chilli powder, garam masala or roasted cumin powder, and salt to taste.
Cook the gravy for 4–5 minutes, stirring occasionally. Add a little water as required if the gravy becomes too thick.
If using cream, stir it in at this stage.
Add the cooked paneer cubes to the gravy and mix gently until well coated.
Cook for another 2 minutes over low heat. Garnish with freshly chopped coriander, if desired, and serve hot.