Heat just half of the oil in a skillet over medium heat. Add the chicken cubes and cook, stirring occasionally, until golden and fully cooked. Transfer to a plate and set aside.

In the same pan, heat the remaining oil. Add the onion, mushrooms, and garlic, and cook for 5-7 minutes, until softened. Return the chicken to the pan. Add the tomato sauce, canned tomatoes, mustard, and ketchup, and cook for 5 minutes longer.

In a small bowl, whisk the cornstarch with a splash of cold water to form a slurry, then stir it into the pan. Turn off the heat and let the mixture cool slightly before stirring in the Greek yogurt. Return to low heat and simmer uncovered for 5-6 minutes, until the sauce has thickened.

Season with salt and pepper to taste and serve.