Heat oil and butter in a pan over medium heat. Sauté the diced veggies, aromatics, green chilies and cashews for 3-5 minutes until they soften.
Turn down the heat and add the spices and dried fenugreek leaves.
Mix and continue cooking for another 2-3 minutes. Take the pan off heat and allow it to cool down.
Transfer to a blender jar.
Blend until you get a smooth sauce that looks like this.
Pour the sauce back into the pan and cook on low for 2 minutes. Add the heavy cream, mix and take off heat.
Notes
Soak the cashews in some warm water before you use them for the recipe. This helps them soften, and they become easier to blend.
Want the sauce to have an even smoother consistency? Once it cools down, pass it through a fine mesh sieve.
Want the sauce to have a smoky flavor? Add a bit of liquid smoke into it. I usually place a small bowl filled with a red-hot coal, topped with a spoonful of Ghee or oil on top of the sauce (making sure it doesn't sink) and then cover it with a lid to let all that smoke infuse into the sauce.
If you don't want to use your blender jar, feel free to use an immersion blender to blitz the sauce. Just make sure you allow it to cool down first.