Drain the chickpeas and combine them in a saucepan with the baking soda and 1¼ L / 5 cups water. Bring to a boil over high heat and then decrease the heat to medium-low and simmer for 30 minutes, until the chickpeas are soft throughout. While cooking, regularly remove the foam that can float on top.
Drain the chickpeas, reserving the cooking water. Reserve a few chickpeas for decorating the dish and transfer the rest to a mixing bowl. Use a hand held blender at low speed to crush the chickpeas into a paste. Add the garlic cloves and continue blending. Slowly add the tahinia, salt, cumin, and lemon juice while blending. Add 4 tablespoons of the chickpea cooking water while blending. If the paste is not smooth, you may need to add up to 1 more tablespoon.
Using a large spoon or a spatula, gently fold in 3 tablespoons of the olive oil until you obtain a creamy, silky consistency. In a small pan, heat the remaining 1 tablespoon of olive oil over medium heat. Add the almond slivers and fry until lightly coloured, about 2 minutes. Drain on paper towels.
Transfer the houmous to a plate and sprinkle the reserved chickpeas, the almonds, and sumac on top. Drizzle with a little olive oil and serve.
