Set baking rack 8 inches below heating element. Turn oven to low broil. Place the onion and ginger halves, cut side up, on a baking sheet. Brush the halves with a little bit of oil. Broil until the onion and ginger slightly charred, about 10 minutes, turning half-way. Remove from heat and cool.
Parboil the beef bones: Fill a large stockpot (about 12 quart capacity) with water and bring to rapid boil. Carefully lower in the bones. Wait until the water returns to a hard boil. Boil vigorously for 3 minutes to release the impurities. Dump bones and water into a clean sink, and rinse the bones with water to wash off any clinging residue. Quickly scrub pot clean and fill with about 6 quarts of clean water. Return bones to pot. Add the charred onion, ginger, spices, beef, fish sauce, and sugar to pot.
Bring water to boil over high heat, and lower heat to gentle simmer. Simmer, uncovered, for 1 1/2 hours.
While simmering, use a ladle or fine-mesh strainer to spoon off any scum that rises on top.
At this point, the boneless meat should be slightly chewy, but not tough. Use tongs to transfer the meat to a bowl. Cover with plastic wrap and refrigerate. Meanwhile, maintain the broth at a steady simmer for 1 1/2 hours longer.
Strain the broth through a fine-mesh sieve (or a coarse-mesh sieve lined with cheesecloth) positioned over a pot. Discard the remaining solids. Use ladle or fine meshed skimmer to skim as much fat as possible from the top of the broth. Taste and adjust the flavor with salt, fish sauce and sugar. There should be about 4 quarts (16 cups) of broth.