Tie the spices in a small packet of cheesecloth.
Heat the tablespoon of oil in a large, heavy-bottomed pot. When it is smoking hot, place the onions and the ginger pieces, cut-side down, in the oil, and cook until they char, 3–5 minutes.
Turn the onions and ginger over and cook for a few minutes on their backs until they start to char.
Add the stock/gelatin mixture, the rest of the stock, the fish sauce, one of the pieces of flank steak, the spice packet, and the sugar to the pot. Bring to a boil.
Reduce the heat to a simmer and stir in the meat-paste mixture. As that mixture cooks, it will rise to the top. This is called a raft. Do not break it up.
Allow the broth to simmer up through the raft, depositing impurities on top and becoming more clear.
Simmer slowly for 45 minutes, uncovered.