• Preheat the oven to 200°F and line a shallow baking pan with foil or parchment.
• If your chicken breasts have tenderloins attached, remove them and reserve for another use. Place the chicken between two sheets of plastic wrap and pound gently to an even 1/2-inch thickness so they cook evenly.
• In a shallow dish, whisk the egg and milk until blended. On a separate plate, combine the cornmeal, breadcrumbs, salt, and pepper.
• Working with one piece at a time, dip each chicken breast into the egg mixture, letting the excess drip off. Press both sides into the cornmeal mixture until evenly coated, pressing lightly so the crumbs adhere well.
• Add about ⅛ inch of oil to a large, heavy skillet and heat over medium-high. When a pinch of crumbs sizzles immediately on contact, the oil is ready.
• Work in batches if needed to avoid crowding the pan and fry the chicken in a single layer until golden and crisp, turning once, 2-1/2 to 3 minutes per side. Check for doneness with an instant read thermometer, and transfer the chicken to the prepared baking pan when they reach an internal temperature of 160°F. Keep warm in the oven while making the gravy.

Make the gravy

• Wipe any excess oil from the skillet, then melt the butter over medium heat. Sprinkle in the flour and whisk until smooth. Continue cooking, stirring constantly, for about 2 minutes, just until the mixture turns pale golden and develops a nutty aroma.
• Add the onion and garlic and cook for 3 to 4 minutes, stirring frequently, until softened. Lower the heat if the mixture begins to brown.
• Whisk in the diced tomatoes with their juices, chicken broth, and milk. Cook, stirring often, until the sauce thickens to a creamy, gravy-like consistency.
• Season to taste with salt and pepper. If the gravy seems too thick, whisk in small amounts of additional broth and milk in equal parts until it reaches your desired texture.
• To serve, spoon a little of the tomato gravy over each chicken breast and pass the rest at the table.