Prepare the Great Northern Beans by soaking them overnight in a brine solution. To do this, add 1 ½ tbsp of salt to 2 quarts of water in a large bowl. Add the beans and cover the bowl. Let the beans soak on the counter overnight. In the morning, drain and rinse the beans.
Add the olive oil to a large stock pot and turn the heat to medium-low. Then the oil is shimmering, add the diced onions and salt. Slow cook the onions until they just begin to brown. About 12-15 minutes.
Increase the heat to medium, then add the garlic. Sauté the garlic and onions for 1-2 minutes being careful not to burn the garlic.
Add the dried thyme and rosemary and stir to combine. Sauté until fragrant. About 1 minute.
Add the tomato paste and continue sautéing until the tomato paste becomes a dark red. About 2-3 minutes.
Add the carrots, celery and diced sweet potatoes. Stir to coat the vegetables.
Add the vegetable broth, 2 cups of water, the soaked Great and the bay leaves.
At this point, you can add the soup to your slow cooker and slow cook it on low for 6-8 hours. Check the slow cooker periodically and add more water as necessary. Or you can continue cooking the soup on the stove top by bringing it to a boil and then reducing it to a simmer. Simmer the soup until the beans are soft. About 2 hours. Be sure to check the soup periodically and add more water as necessary.
As the liquid cooks down, the soup should become very thick and flavorful. Add additional water to achieve the desired consistency. Remove the bay leaves and add the sherry vinegar. Serve immediately season with salt & fresh ground pepper, to taste.