In a large skillet over medium-high heat, combine 1 tablespoon of oil, diced onion, and garlic. Once onions have softened and garlic is fragrant, add in tomatoes, curry powder, garam masala, paprika, chili powder, cumin, and turmeric. Lower the heat to medium and cook about 10 minutes, occasionally stirring.
Remove the sauce from the pan and transfer to a food processor. Blend until smooth with the 2 tablespoons of water. Be careful as the sauce will be very hot.
In the same skillet used for the sauce, place the remaining oil and chicken over medium-high heat. Season with a little salt and pepper. Cook until the chicken is almost completely cooked through before adding the sauce back into the pan. Stir in the Greek yogurt and until everything is well combined and smooth before reducing the heat to medium-low for 10 minutes.
Serve with cilantro as garnish over either rice or with naan bread.