Dip tomato slices in egg; dredge in breadcrumbs. Fry the tomato slices in hot bacon drippings in a large skillet until browned, turning once. Drain well on paper towels. Arrange on serving platter; set aside and keep warm
Reserve bacon drippings in skillet. Add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in mustard, salt, and pepper. Spoon gravy over tomatoes to serve.