Rinse the dried white beans thoroughly under cold water, then pick out any debris or bad beans. Set aside.
Dice the large yellow onion into small pieces and peel and mince the garlic cloves. These will build the soup’s flavor base.
Add the rinsed beans, diced onion, minced garlic, dried thyme, and a pinch of salt and pepper into the slow cooker. Pour in the vegetable broth, covering everything generously.
Cover the slow cooker and set it to low for about 8 hours. During the first 4 hours, the beans will start to soften and release a fragrant aroma of garlic and herbs. You may hear gentle bubbling as the beans cook slowly.
Once the cooking time is up, open the lid and check the beans. They should be silky, breaking apart easily. Use a wooden spoon to mash some of the beans directly in the pot to thicken the soup and enhance its creamy texture.
Stir in a tablespoon of olive oil and a squeeze of fresh lemon juice to brighten the flavors. Taste and adjust seasoning with more salt or pepper if needed. Let the soup sit for 10 minutes to allow the flavors to meld.
Ladle the hot soup into bowls, drizzle with a little extra olive oil if desired, and serve with crusty bread or a simple green salad. The soup should be velvety, fragrant, and comforting, perfect for a cozy evening.