Roast the beef ribs in a 325 degree oven for around 30-40 minutes. You want them to brown up a bit.
Bruise the lemongrass. Cut it in half and whack it repeatedly with the dull side of a big knife until it starts to break open.
Combine the roasted ribs (or leftover ribs from a rib roast), lemongrass, chicken stock, star anise, black cardamom, shallots and fish sauce in a pot large enough to hold all the ingredients.
Bring to a simmer and cook until the beef separates easily from the bone. This takes 90-120 minutes or so.
This next step is optional. If you like the textured bits in pho (like tendon) then go for it. If you prefer just sliced beef then leave the meat on the bones and continue simmering. Your broth will be beefier for it.
Remove the ribs from the pot. Let cool slightly. While you are at it remove everything except the shallots from the pot. Once the ribs have cooled enough to handle remove the meat and refrigerate. Return the stripped beef bones to the pot and continue simmering for a total of 4 hours. If your beef took 2 hours to get tender simmer for another 2 hours. Or, if you don't want the more interesting bits then just skip this step. Leave the beef on the bones but do remove the lemongrass, star anise and cardamom.
After 4 hours, remove the beef bones. Strain. Add enough water or stock to bring the total volume back to 6 cups. Refrigerate briefly. You want the fat to set up. There's too much at this point. Once the stock is chilled skim off the fat. Keep it handy though. Don't throw it away.
Return the skimmed stock to the pot. Bring to a simmer. Taste and adjust for salt.
Add three tablespoons of fat to a small sauce pan or frying pan. Heat over medium low heat. It will sputter for a bit as the water boils off. You should have enough fat. If you don't make up the difference with vegetable oil.
Add the bun bo Hue spice to the pan. Stir to coat the spices in oil and fry gently for about 30 seconds. Let this cool briefly and add about a tablespoon of the spice/fat mixture to the broth. Be careful. Hot fat into stock can bubble a bit. Taste and decide if it's spicy enough. If not, add a bit more and taste again. Once you have the salt and spice right your broth is done. Keep warm.