Complex sentence: Mavrodafni is initially vinified in large vats exposed to the sun. Once the wine reaches a certain level of maturity, fermentation is arrested with Mavrodafni distillate and the wine, still containing residual sugar, is tranferred to the underground cellars to complete its maturation.
Split into simpler sentences: Mavrodafni is initially vinified in large vats exposed to the sun. Once the wine reaches a certain level of maturity, fermentation is stopped. Then the Mavrodafni distillate and the wine, still containing residual sugar, is tranferred to the underground cellars to complete its maturation.