Place garbanzo beans in a large saucepan; add water to cover by 1 in. Gently rub beans together to loosen outer skin. Pour off water and any skins that are floating. Repeat 2-3 times until no skins float to the surface; drain. Return to saucepan; add baking soda and enough water to cover by 1 in. Bring to a boil; reduce heat. Simmer, uncovered, until beans are very tender and just starting to fall apart, 20-25 minutes., Meanwhile, in a blender, process lemon juice, garlic and salt until almost a paste. Let stand 10 minutes; strain, discarding solids. Stir in cumin. In a small bowl, stir together tahini and olive oil. , Add beans to blender; add cold water. Loosely cover and process until completely smooth. Add lemon mixture and process. With blender running, slowly add tahini mixture, scraping sides as needed. Adjust seasoning with additional salt and cumin if desired., Transfer mixture to a serving bowl; cover and refrigerate at least 30 minutes. If desired, top with additional olive oil and assorted toppings.