Add the Yellow Onion (1), Garlic Paste (1 teaspoon), and Ginger Paste (1 teaspoon) to a blender and pulse until it is pureed. Keep half of it aside in a small container for the gravy.
Take a large mixing bowl and mix the Red Chili Powder (1 tablespoon), Garam Masala (1 teaspoon), Ground Turmeric (1/2 teaspoon), Ground Coriander (1/2 teaspoon), Ground Cinnamon (1/2 teaspoon), and Ground Cumin (1/2 teaspoon) together with half of the onion, garlic and ginger puree.
Add 1 teaspoon of salt and a few grinds of fresh pepper, the Yogurt (1/2 cup), Lemon Juice (2 tablespoon) and combine everything well with a spoon until it is a thick marinade.
Pop the Chicken Thigh (1 kilogram) into the bowl and coat each piece with the marinade evenly. Place a lid or cling wrap over the top of the bowl and refrigerate. It is best to do this step the day before so it has time to marinate overnight. If you don’t have time a few hours will do.
Preheat your oven to 350 degrees F (180 degrees C) when you are ready to prepare the butter chicken in its entirety.
Line two roasting trays with aluminum foil and place the marinated diced chicken on top, spreading it apart so it can cook evenly. Place the trays in the oven and cook for about 30-40 minutes or burnished golden brown on the edges. In between, you may need to turn the chicken pieces so they color evenly and switch the position of the trays.
In the meanwhile, you can prepare the gravy by heating Butter (1 1/2 tablespoon) in a medium to large saucepan. Add the rest of the onion, ginger, and garlic puree and sauté for 2-3 minutes until fragrant. Place the Whole Cardamom Pods (3) pods and allow them to flavor.
Next add in the Tomato Passata (600 milliliter) followed by the Red Chili Powder (1 teaspoon), Garam Masala (1/2 teaspoon), Ground Cinnamon (1/2 teaspoon), Caster Sugar (1/2 teaspoon), and simmer for 8-10 minutes. When you see the oil begin to separate you will know the gravy is ready.
Add the Thickened Cream (100 milliliter) and Dried Fenugreek Leaves (1/2 teaspoon) stirring thoroughly. Taste the gravy and season accordingly, adding more salt and pepper if necessary. If you find the gravy to be too spiced, add a bit more cream.
Add in the cooked chicken pieces, stirring through well, and then the remaining Fresh Cilantro Leaf (1 tablespoon). When serving drizzle some Butter (1 1/2 tablespoon) and Thickened Cream (to taste).