If using fresh spinach, blanch it for 1 minute in a large pot of boiling water, and then use a large strainer to transfer it to a large bowl of ice water to stop the cooking and set the color.
Squeeze out the excess water, place the spinach in a blender, and purée it; set aside.
If using frozen spinach, once it has thawed, squeeze it gently to remove any excess liquid.
Heat a large skillet and, when it’s hot, add the butter and olive oil.
Add cumin seeds, onion, and chilis and cook for 5 minutes or until the onions are light brown.
Add ginger garlic paste, garam masala, cayenne, salt, and tomatoes and cook for another 5 minutes.
Add the chicken and cook until firm and cooked through, 5 to 8 minutes, flipping the pieces halfway through.
Add the nutmeg and fenugreek and stir for 2 minutes, then add the puréed spinach and a cup of water, and cook for another 5 minutes.
For a creamier sauce, add the heavy cream and cook 3 to 5 more minutes.