Add a tablespoon of butter and a little oil to the same pan. Let the onion soften over low heat. Once it turns nicely golden, add the diced tomatoes and cashews and sauté everything together briefly.

Pour a little water into the pan mixture. Add the garlic, salt, white vinegar, sugar, garam masala and Kashmiri chilli. Stir well, cover, and let it simmer gently for about 15 to 20 minutes.

Transfer the cooked mixture to a blender (add a splash of water if it seems too thick) and blend until completely smooth. To make the sauce perfectly velvety, pass the purée through a fine sieve back into the pan.

Return the seared chicken to the smooth sauce in the pan. Add butter, whipping cream and a teaspoon of dried kasoori methi (fenugreek leaves). Simmer together for another 5 to 7 minutes so all the flavours come together.

Taste the sauce. If you'd like it milder, stir in a bit more cream. Serve the finished butter chicken garnished with a swirl of cream and a few kasoori methi leaves. It's best with naan bread or basmati rice.