Beat together egg and water in a small bowl. Place bread crumbs in a shallow dish.

Dip eggplant slices in egg mixture, then press in bread crumbs to coat.

Heat oil in a large skillet over medium-high heat. Add eggplant slices to hot oil, then reduce heat to medium. Cook until golden brown and tender, 3 to 4 minutes per side. Sprinkle mozzarella cheese over eggplant during the last minute of cooking to melt.

While eggplant is cooking, combine spaghetti sauce and red pepper flakes in a microwave-safe measuring cup. Cover with plastic wrap and cook on high until heated through, about 2 minutes.