Clean the Bones: Place bones in a large pot, cover with water, and bring to a rolling boil for 5–10 minutes. This forces out grey scum and impurities. Drain and rinse the bones thoroughly under cold water to ensure a crystal-clear broth later.
Pressure Extraction: Transfer cleaned bones to an Instant Pot with 1–2 tsp of salt. Cover with water and cook on Manual High Pressure for 60 minutes. Allow a natural release for 20 minutes to keep the broth clear; a quick release can agitate the fat and make the liquid cloudy.
Char the Aromatics: While the bones cook, place the halved onion and ginger directly over a gas flame or under a broiler until the skins are blackened and the insides are softened. Do not peel the char off; it provides the deep golden hue and smokiness.
Toast the Spices: In a dry pan over medium heat, toast the star anise, cinnamon, cardamom, coriander, cloves, fennel, and peppercorns for 2–3 minutes until they become highly fragrant. Place them in a spice sachet or cheesecloth.
Stovetop Infusion: Transfer the Instant Pot broth and bones into a large stockpot. Add the flank, brisket, charred onion, ginger, and spice sachet. Top with more water and bring to a gentle simmer (never a rolling boil).
Gentle Cook & Skim: Simmer for 75–90 minutes. Periodically skim off any rising foam. If you prefer a leaner broth, skim the fat, but leave some for a rich mouthfeel.
The "Ice Bath" Shock: Once the flank/brisket is tender, remove it along with the tendon. Immediately submerge them in a bowl of ice water for 5–10 minutes. This stops the cooking and prevents the meat from oxidizing and turning dark.
Final Seasoning: Taste the broth. Adjust with fish sauce, rock sugar, salt, and chicken bouillon until it tastes savory, slightly sweet, and deeply beefy.
Prepare Noodles: Boil the dried rice noodles in a separate pot for 8–9 minutes (or according to package directions) until al dente. Drain and rinse briefly in cold water to remove excess starch.
Assembly: Divide noodles among bowls. Top with slices of cooked meat and paper-thin slices of raw eye of round. Ladle the boiling broth directly over the raw beef to cook it instantly. Garnish with onions and herbs, and serve with the side plate and sauces.