Heat oven to 450° F. Place 1-1/2 cups Dixie Fry Mix in shallow baking dish. Beat egg and buttermilk together in bowl. Whisk remaining 1/2 cup Dixie Fry Mix into buttermilk mixture.
Working one at a time, dredge each onion ring in Dixie Fry Mix, shaking off excess. Dip rings in buttermilk mixture, allowing excess to drip back into bowl, then drop into Dixie Fry Mix, turning rings to coat evenly. Transfer coated rings to large plate or tray.
Pour 3 tbsp oil onto rimmed baking sheet. Place baking sheets in oven and heat until just smoking, about 8 minutes. Carefully tilt heated sheets to coat evenly with oil, then arrange onion rings on sheets. Baking, flipping onion rings and switching and rotating position of baking sheets halfway through baking, until golden brown on both sides, about 15 minutes.