In a medium bowl, cover the chickpeas with enough water to reach 2 inches above the height of the chickpeas and soak them for 12 hours.
Drain and rinse the chickpeas and place them in a large saucepan with the baking soda and 1/2 teaspoon of the salt. Cover them with 1 to 2 inches water and bring the water to a boil over high heat. Reduce the heat to low, cover the pot and simmer until the chickpeas are very soft, about 2 hours. Drain them, let them cool slightly and then transfer to a food processor.
Add the lemon juice, tahini, 1/4 teaspoon salt and the garlic, if using, and blend until very smooth, about 1 minute. With the motor running, drizzle in the water and continue to blend for 1 more minute. Taste and add additional salt to taste. Transfer to a serving dish and top with a drizzle of olive oil and some chopped onion and parsley. Serve with fresh pita and onion wedges and enjoy.
Any leftovers should be stored in an airtight container in the fridge with a thin layer of olive oil coating the top.