This is my entirely non-authentic version, but sweet jeezus it was delicious!
Put bone and leftover fat and bits from a sirloin roast beef (it's what I had, leftover from
skipdiver's birthday dinner), an onion, a big carrot, couple of celery stocks, bay leaf (fresh off my tree), clove of garlic, two sticks of lemongrass, salt into a big pot with a ton of cold water. Boil it up into a stock for a few hours, let it cool overnight or for a few hours, skim off the fat. Boil again for an hour or two or three with a stick of cinnamon, a thumb-size chunk of ginger, a star anise, couple of cardamoms, 4-ish whole cloves, handful of fresh coriander, glug of fish sauce, one birdseye chilli, de-seeded. Strain it and set aside. Fry some onions, then lightly stir fry other veg you have lying around - I did thinly sliced red bell pepper, bok choy, and grated carrot. Prepare rice noodles ever so slightly underdone. Put noodles, veg, bean sprouts, fresh coriander, mint and basil, and slices of roast beef (I used the sandwich meat packaged kind, sliced into strips) into the bowl, bring the broth to the boil, pour into the bowl over the stuff, squeeze in half a lime or more to taste. Can also add a de-seeded birdseye chilli, and anything else you want (more fish sauce, chilli sauce, whatever - I just did the lime juice and birdseye chilli). Absolutely amazing! It's all about the broth, though.