Chef César Gachupin de Díos and his son, Victor Gachupin Velasco, prepared freshwater seafood soup with hot stones in a pre-Hispanic method from Oaxaca during a recent culinary residency at Vaughan’s Lounge in Bywater. Sarah Borealis co-produced a documentary, “The Path of Stone Soup,” being screened at the residency, and guests sampled the broth.