* Roasting the bones prior to cooking is also an option. I have not tried it, but I suspect it can give a deeper flavor.
** Don't mess with the pot contents while cooking! Leave them alone! Don't cloud up the broth. You want to keep it clear and light. Keeping the broth at a bare simmer helps with this too. If the onion or beef seem to be disintegrating while cooking, gently take them out. UPDATE: The chicken feet will the cloud the broth slightly, but that's a sacrifice I don't mind making.
*** I love tripe and tendon in my pho. I can't eat it without including these two ingredients. If using, let the butcher pre-cut them into bite-sized pieces before cooking. They are like cutting rubber bands, so let someone else do the work. Many Asian markets sell tendon and tripe frozen and pre-cut in cubes, which are the perfect size for eating in pho. If not cooked enough, tripe and tendon are like eating rubber. If cooked perfectly, tendon is buttery and rich, the tripe with a bit of crunch without being too chewy. They add great texture. Tripe should be pre-cooked when you purchase it, so a rinse and quick boil and simmer should be okay before adding to bowls to serve. The tendon requires more work; see the recipe steps for details.