In a large bowl, combine the chicken pieces with ginger-garlic paste, chili powder, turmeric, chicken masala, black pepper, vinegar, tomato paste, and salt. Mix well to coat all pieces. Cover and refrigerate for at least 1 hour.

Heat olive oil in a large skillet over medium heat. Add the marinated chicken, cover, and cook for 15 minutes, stirring occasionally.

Remove the lid and increase heat to medium-high. Cook for another 5 minutes until excess liquid evaporates and chicken develops light golden edges.

In a separate saucepan, melt butter over low heat. Add flour and whisk for 1 to 2 minutes until fragrant but not browned.

Gradually pour the warmed milk into the roux while whisking constantly. Continue whisking as the mixture heats and thickens, about 3 to 4 minutes, until it coats the back of a spoon.

Stir the oregano, chili flakes, salt, and pepper into the white sauce. Remove from heat.

Pour the white sauce evenly over the cooked chicken in the pan. Sprinkle mozzarella cheese and chopped cilantro over the top.

Cover the pan and cook on low heat for 3 to 5 minutes until the cheese melts completely. Optional: place under broiler for 1 to 2 minutes for golden brown spots on cheese.

Serve hot directly from the pan with Turkish pide, naan, or rice.