Rinse the garbanzo beans and and place in a pot. Fill with enough water to cover by at least 1 inch. Add baking soda, if using. Bring to a boil and then simmer over medium heat until the beans are very soft, 1 1/2 to 2 hours.
Drain the beans, reserving some of the water to use later. Reserve a small handful of the whole beans for a garnish. Transfer the rest to a blender or if you have a hand blender, a large bowl. Blend the beans until smooth, adding 1/2 cup of olive oil gradually. Add some of the reserved water if needed to help it blend. Add the tahini and blend in along with the lemon juice. Blend in the garlic, cumin and salt.
Spread the hummus into a flat serving dish and garnish with the reserved beans and a drizzle of olive oil.
