You must NOT omit the sour cream in this recipe. I have already done the testing and failed with just Greek yogurt in the sauce. Since the Greek yogurt contains zero (or low) fat, it will curdle as soon as you add it to the hot mixture (since we aren't taking the time to temper it). To ensure that it doesn't curdle, you must mix the sour cream with the Greek yogurt. Sour cream contains fat and this fat will prevent the Greek yogurt from curdling and separating. If you have experience cooking with Greek yogurt and are comfortable with omitting the sour cream and replacing it with all Greek yogurt, by all means, go for it. However, I cannot promise you success as I have done this twice and my sauce has separated. The only success I got was when I mixed my Greek yogurt with sour cream.