Any pho aficionado worth their salt will tell you that the broth—not the noodles, meat, or herbs—is key to the dish's success. Hours of simmering are typically required to produce an ideal broth that is both savory and clear. _Viet World Kitchen_ specifies that even the bones chefs choose to simmer are important: "Avoid neck bones. Look for knuckle bones and leg bones that contain marrow." Charred onion, ginger, star anise, cinnamon, and other seasonings then give the soup its nuanced taste, but what you add to the mix from there, whether it's beef tendon or tofu, is entirely up to you.