Native Americans mixed beans, maple sugar, and bear fat in earthenware pots which they placed in underground "bean holes" which were lined in hot rocks to cook slowly over a long period of time. British colonists in New England were the first westerners to adopt the dish from the Native peoples; and were quick to embrace it largely because the dish was reminiscent of peas porridge and because the dish used ingredients native to the New World. They substituted molasses or sugar for the maple syrup, bacon or ham for the bear fat, and simmered their beans for hours in pots over the fire instead of underground. Each colony in America had its own regional variations of the dish, with navy or white pea beans used in Massachusetts, Jacob's Cattle and soldier beans used in Maine, and yellow-eyed beans in Vermont. This variation likely resulted from the colonists receiving the dish from different Native peoples who used different native beans. While some historians have theorized that baked beans had originated from the cassoulet or bean stew tradition in Southern France, this is unlikely as the beans used to make baked beans are all native to South America and were introduced to Europe around 1528. However, it is likely that English colonists used their knowledge of cassoulet cooking to modify the cooking technique of the beans from the traditional Native American version, by soaking the bean overnight and simmering the beans over a fire before baking it in earthen pots in order to decrease the cooking time. Indeed, a dish which was a clear precursor to baked beans, entitled "beans and bacon", was known in medieval England. The addition of onion and mustard to some baked beans recipes published in New England in the 19th century was likely based on traditional cassoulet recipes from Staffordshire, England which utilized mustard, beans, and leeks. These ingredients are still often added to baked beans today. Nineteenth century cookbooks published in New England, spread to other portions United States and Canada, which familiarized other people with the dish. While many recipes today are stewed, traditionally dried beans were soaked over night, simmered until tender (parboiled), and then slow-baked in a ceramic or cast-iron beanpot. Originally baked beans were sweetened with maple syrup by Native Americans, a tradition some recipes still follow, but some English colonists modified the sweetening agent to brown sugar beginning in the 17th century. In the 18th century the convention of using American made molasses as a sweetening agent became increasingly popular in order to avoid British taxes on sugar.