Start by roasting the beef bones in the oven until they are browned and aromatic. This step helps to enhance the depth of flavor in the broth.
In a large stockpot, add the roasted beef bones, along with sliced onions, ginger, and the aromatic spices such as star anise, cloves, cinnamon, cardamom, and coriander seeds.
Fill the pot with water, making sure that the ingredients are fully submerged. Bring the mixture to a boil over high heat, then reduce the heat to low and let it simmer for at least 4-6 hours. This slow simmering process allows the flavors to develop and the collagen from the bones to break down, resulting in a rich and gelatinous broth.
As the broth simmers, skim off any impurities that rise to the surface. This will help to ensure a clear and clean broth.
After the simmering time is up, strain the broth through a fine-mesh sieve to remove any solids. The result should be a clear and flavorful beef broth, ready to be transformed into a bowl of pho.