Cover the beans with water and allow to soak overnight. The next morning, drain the beans and rinse. Cover with fresh water, bring to a boil, reduce heat to a simmer and cook 30 minutes. Drain off the bean water and reserve. Mix the beans with the salt, mustards, and maple syrup. Place the peeled onion in the bottom of a 3-quart casserole dish; pour the bean mixture over the onion, and cover with the reserved bean water. Place the strips of bacon over the top. Bake, covered, in a 325°F oven for three to four hours, or until the beans are very soft. Add more water if the mixture becomes too dry. Bake uncovered for the last hour in order to brown the bacon.