Heat oil over medium heat in a medium saute pan. Add onion and cook for 3-4 minutes. Add garlic and cook for an additional minute. Transfer to bowl of slow cooker.
Quarter potatoes and add to slow cooker bowl. Add salt, pepper, nutritional yeast (if using), and vegetable broth. Stir well.
Cook on low for 4-5 hours. Once done, use potato masher to mash potatoes. Or for a smoother soup, use an immersion blender to blend until smooth. Serve topped with bacon pieces and green onion slices.