Place the Greek yogurt in a large bowl. Add the lemon juice, cumin, garam masala, chili powder, salt, grated ginger, and garlic.
Mix until the marinade is smooth and evenly combined.
Add the chicken pieces and toss thoroughly so they are well coated on all sides.
Cover the bowl and refrigerate for at least 30 minutes. If possible, let the chicken marinate for a few hours.
Heat about half of the vegetable oil in a large skillet over medium-high heat.
Remove the chicken from the marinade and arrange the pieces in the skillet without overcrowding. If necessary, cook the chicken in two batches.
Brown the chicken for about 5-6 minutes, turning it during cooking so it colors on several sides. It does not need to be fully cooked at this stage, as it will finish cooking in the sauce.
Transfer the chicken to a plate and set aside.
Add the remaining oil to the same skillet, then add the chopped onion. Cook over medium heat for about 4-5 minutes, stirring often, until softened and lightly golden.
Add the ginger and garlic and cook for about 30 seconds.
Add the brown sugar, ground coriander, smoked paprika, and garam masala. Stir for a few seconds, allowing the spices to toast lightly without burning.
Pour in the tomato passata and stir well to combine with the spiced base.
Cook over medium-low heat for about 4-5 minutes, stirring occasionally, until the sauce begins to reduce slightly.
Add the heavy cream and stir until the sauce is smooth and creamy.
Add the drained chickpeas and browned chicken, along with any juices left on the plate.
Let everything simmer gently until the chicken is fully cooked through, about 8–10 minutes. If you're using a food thermometer, the thickest pieces should reach 165°F (74°C). The sauce should be creamy and well combined.
If the sauce becomes too thick during cooking, add a little water at a time until you reach the desired consistency.
Taste and adjust the salt if needed.
Serve the chicken & chickpea tikka masala hot and, if desired, garnish with fresh cilantro or parsley and serve with flatbread.