Wash and dry vegetables, herbs, beef heel muscle, and beef bones. Cut lime and chili pepper. Chop scallions and cilantro (leaving roots intact). Halve one onion, thinly slice the other and smash lemongrass stalks.
Add the lemongrass and scallions to a pot of boiling water. Boil the water together with the aromatics for 3 minutes.
Add the beef bones and beef heel muscle to the pot. Cook the beef for 3 minutes. Transfer the ingredients to a bowl of cold water and clean them well. Bake the beef bones, the onion that you cut in half, shallots, and ginger at 400 degrees F for 15 minutes.
Add the aromatics and cilantro roots to a heated pan. Roast them for 2 to 3 minutes or until fragrant.
Wait for the spices to cool, and add them to a spice bag. Soak the spice bag in warm water for 30 minutes.
Pour water into a pot and add the beef bones, beef heel muscle, onions, shallots, and ginger. Stew for 2 hours over medium heat. Season the broth with a little salt and rock sugar.
Skim the broth frequently. At the 30-minute mark, add the spice bag to the pot.
After 1 hour and 30 minutes, remove the beef heel muscle from the pot. Put the beef in cold water to prevent browning. Add the beef balls and cook them for 15 minutes on low heat.
Remove all ingredients except for the beef bones from the pot.
Season the broth with more rock sugar and soup powder.
Slice the beef heel muscle and cut the beef balls in half.
Blanch the phở noodles in boiled water. Add a little cooking oil to prevent the noodles from clumping.
Arrange the accompanying herbs and condiments on different plates. In a bowl, top noodles with beef slices and balls, then pour over broth. Garnish with scallions, cilantro, sliced onions, pepper, and chili (optional).