Rinse the beef bones with salt, place them in a pot, cover with water, and bring to a boil for 5 minutes. Turn off the heat, drain, and rinse the bones. Discard the boiling water. Return the bones to the pot, add just enough water to cover, and simmer on low heat to keep the broth clear. Skim off any foam, then add a little salt and rock sugar to the pot.

Toast the pho spices in a hot pan until fragrant, then place them in a metal spice bag and add to the pot with the bones.

Char the onion and ginger until the skins are slightly burnt and fragrant. Peel off the burnt skins, rinse, and lightly crush. Add them to the broth. Season the broth with salt, rock sugar, and fish sauce to taste.

Blanch the pho noodles in boiling water to heat them, then place them in bowls. Arrange the sliced beef sirloin on top and pour the boiling broth over the beef to cook it gently without making it tough. Sprinkle with green onions and black pepper. Serve pho with bean sprouts and fresh herbs.