Put 2 tablespoons (30 ml) of the water and the onion in a medium saucepan. Cook over medium heat, stirring occasionally, until the onion is soft, about 5 minutes. Add a little more water if needed to prevent sticking.
Put 1 cup (250 ml) of the water and the chickpeas, tamari, and garlic in a blender and process until smooth. Add the remaining water and the cornstarch, parsley, sage, ros