In a soup pot or Dutch oven, place the broth and water over high heat. Add the lima beans and cover.
Peel and chop the onion and add it to the pot. Add the salt, pepper and garlic.
Keep the pot covered until the mixture comes to a boil. Then uncover and stir. Cover the pot and cook on medium-high, keeping a steady boil for 10 minutes, or until the beans are tender.
Remove the soup from the stove. Using an immersion blender, puree the soup until it is smooth (or you can use a potato masher to do this). Place the pot over medium heat.
In a small jar that has a lid, add the cornstarch and 1/4 cup cool tap water, and shake to mix well. Drizzle the cornstarch mixture into the soup, and stir constantly until the soup thickens, about 2 minutes.
Serve immediately, garnished with bacon bits, sour cream and parsley, if desired.