Rinse the beans in a sieve under running water. Dump them into the slow cooker and add the stock and crushed red pepper. Cover and cook on low for 8 hours.
Ladle out and discard excess liquid, if there is any—the stock should just cover the cooked beans. Stir the tahini into the soup. With an immersion blender, puree about half of the beans to thicken the soup. Season with salt and plenty of black pepper.
In a small sauté pan, heat the oil over medium heat. When it shimmers, add the garlic and cook, stirring, until golden, about 3 minutes. Scrape the hot garlic oil into the soup. Serve.