Complex sentence: The Welsh were also early adopters of roasting cheese, an early incarnation of Welsh rarebit was being made in medieval times, and by the middle of the 15th century Rarebit was considered a national dish.
Split into simpler sentences: The Welsh were also early adopters of roasting cheese. An early incarnation of Welsh rarebit was being made in medieval times, and by the middle of the 15th century Rarebit was considered a national dish.