Start by preparing the marinade. In a bowl, combine yogurt, minced garlic, grated ginger, lemon juice, apple cider vinegar, vegetable oil, salt, cumin, coriander, paprika, garam masala, fenugreek, and cayenne pepper. Mix well to form a smooth paste.

Place the chicken pieces into the marinade. Ensure every piece is well-coated. Cover the bowl with plastic wrap and refrigerate for at least 1 hour, but ideally overnight for maximum flavor absorption.

While the chicken is marinating, prepare your sauce. Heat vegetable oil in a large saucepan over medium heat. Add the diced onions and cook until they soften and turn golden brown, around 5-7 minutes.

Add the grated ginger and garlic to the onions. Stir and cook for an additional minute until fragrant. Then, add the sweet paprika, salt, and cook for 2 minutes more.

Stir in the garam masala, turmeric, coriander, cumin, cayenne, and cardamom. Cook the spices for about 1-2 minutes. Don't rush; let them bloom for a better flavor.

Add the tomato passata and water, stirring well to combine. Bring to a simmer, and then reduce the heat to low. Cook for about 15 minutes to thicken the sauce.

Stir in the heavy cream and sugar to balance acidity. Allow the sauce to simmer gently for around 5 minutes while you prepare the chicken.

Preheat your grill. Lightly oil the grates to prevent sticking. Grill the marinated chicken pieces over medium-high heat for 5-8 minutes per side, or until cooked through and slightly charred.

Once the chicken is grilled, transfer it to the saucepan with the sauce. Gently fold the chicken in to coat it in that creamy goodness. If desired, finish with additional butter for richness.

Spoon the chicken tikka masala onto plates and serve with naan, rice, or your preferred sides. Garnish with fresh cilantro for an added touch of color.