Add oil and butter to your pan and heat it on medium heat.
Once the butter has melted, add in your onions and cook for about 4 minutes before removing them.
Add in your chicken, in batches of 4, and fry for 2-3 minutes per side. Add more oil or butter as needed.
Once all the batches are done, remove your chicken from your pan.
Add in your crushed tomatoes, cooked onion, remaining marinade, 1/2 cup cream, garlic paste, ginger powder, salt, chili powder, garam masala, coriander, cumin and mix well.
Cook your sauce for 15 - 20 minutes on medium low heat.
Turn off the heat and add your sauce to a blender.
Strain your sauce back in your pan.
Add your soaked cashews to your blender along with 1/4 cup fresh water and blend until smooth. Strain the mixture back in the pan.
Turn back on the heat and add in your sugar, fenugreek leaves and the remaining cream to your sauce and mix well.
Toss in your chicken, ensuring it is well coated in the sauce. Cover and cook for 10 minutes.
Drizzle 1/2 tbsp cream over the sauce and serve with naan or basmati rice.