Soak the beans overnight in a large bowl filled with water.
*Soooooooooooo...if you didn't think yesterday to soak your beans you can do the "quick soak" method. Put the beans in a large pot and cover with at least 2 inches of water. Bring the beans to a boil over medium heat. Turn the heat off and let the pot sit until the water cools down, then proceed as if you had soaked the beans overnight.
Strain the beans from the soaking water. Put the beans in a large pot and cover generously with water. Add the onions, carrot, celery, garlic, thyme, bay and slab bacon Bring the pot to a boil over high heat. Reduce heat and simmer until all the beans are soft, about 1 hour. Add more water if it reduces too much. Test the beans for doneness by doing the 5 bean test. Bite into 5 beans, if they are all soft then they are done. Beans do not all cook at the same rate so the 5 bean test is important.
When the beans pass the 5 bean test, remove the pot from the heat. Season the water generously with salt and let sit for 15 minutes. If using the beans right away, go ahead. If not using the beans right away, store them covered, in the refrigerator in their liquid.
Beans, beans the musical fruit...
Due to the soaking this is definitely a plan ahead recipe.