In a large pot, combine the beans and enough water to cover by 2 inches.
Bring to a boil over high heat, and boil for 2 minutes.
Remove from the heat, cover the pot, and let stand for 1 hour.
The beans can also be soaked overnight in a large bowl with enough cold water to cover by 2 inches.
After soaking the beans, drain well.
Return the beans to the pot and add enough fresh water to cover by 2 inches.
Add the halved onion and bring to a boil over high heat.
Reduce the heat to low and simmer until the beans are almost tender, about 1 hour.
Drain well, reserving 1 cup of the cooking liquid.
Discard the onion.
(The beans can be prepared a day ahead; cover and refrigerate the beans and the cooking liquid separately.)
In a 3 1/2 quart slow cooker, combine the salt pork, reserved 1 cup cooking liquid, the chopped onion, maple syrup, spicy mustard, catsup, and dry mustard.
Stir in the drained beans.
Cover and slow cook until the beans are just tender, 7 to 8 hours on low heat.
Increase the heat to high, uncover, and cook until the excess liquid has evaporated, about 1 hour.