Add more butter to the same pan used for cooking the chicken and let it melt.
Add the chopped onions and cook until they turn brown.
Then, add the tomatoes, garlic, ginger, chilies, and the seasonings. Mix everything and cook for about a minute.
Add the cashews and cream and cook for a few more minutes to soften them.
Pour the mixture into a blender and blend until smooth. Sieve the sauce back into the pan and add the cooked chicken.
Let it simmer over medium heat for about 5 minutes to thicken the sauce.
If the butter starts separating, add more cream and mix.
Finally, add the garam masala and coriander, remove from heat, cover, and leave aside.