Heat a little olive oil in a large shallow pan on medium-high heat. Brown the sausages (leave them whole, slice them, or crumble them). Remove the sausages and place them in the crockpot.
In the same pan, add the chopped onions and garlic and saute for about 2–3 minutes, or until the onions have softened a little. Transfer to the slow cooker.
Add the rest of the ingredients (tomatoes, balsamic vinegar, dried thyme, bay leaves, and white beans) and stir. Cover and cook on high for 2-3 hours or 4-5 hours on low.
I use organic cannellini beans (from Trader Joe's), and I don’t rinse them for this dish because the extra liquid adds flavor and helps thicken the stew.