pho broth has some sweetness to it. The recipes I've seen recommended rock candy. There is also clove, anise, coriander, cardamon, fennel and cinnamon. Along with some onion.
You want it to be very aromatic and with a deep rich savory beef goodness. Bone broth is really good for it because of how rich it is. But regardless, pho broth is almost always beef based.
I realize. I'm with Siro in that I haven't found good pho. The last place I tried was Pho 99 on Redwood and the broth was so bland that it was a constant experiment adding various spices and soy to get a reasonable taste. The bowl seemed big enough for three but what do portions matter when the taste is poor.
My typical broth is made from ~18lbs beef shin bones, 4lbs ox tail, a few pounds beef spare ribs. Start by putting quartered onion on a screen directly over a gas burner to get a good char and roasting the bones until they have some nice roasted color. Add to stock pot and cover with cold water. Add about a cup of chinese black vinegar and a handful of whole black tellicherry pepercorns. Simmer for about 48 hrs, skimming and saving the rendered fat along the way. I use the rendered fat (tallow) for cooking.
Have you used this for pho?
I may start making stock. I've read that using multiple areas of bone is better than just one kind.