In a shallow bowl, whisk the egg and water to make an egg wash. Add the panko bread crumbs and flour to two separate shallow bowls (I usually season each with a little salt + pepper). Dredge the eggplant slices in flour, then dip in the egg wash and finally coat with the bread crumbs.
Heat 1/4" frying oil in a 12" cast iron skillet on high. When the oil is just beginning to smoke, add the eggplant slices and slightly lower the temperature (you might need to do two batches). Cook for about two to three minutes per side, or until golden brown. Transfer to a paper towel lined plate and immediately season with salt.
Plate them up with the tomato sauce and enjoy immediately.