Set a 6-qt. Instant Pot to high sauté. Toast 3 star anise pods, one 3″ cinnamon stick, and 4 whole cloves, stirring, until fragrant and crackling slightly, about 2 minutes. Add 2 Tbsp. vegetable oil and work around to coat bottom of pot. Reduce heat to medium, add 2 medium yellow onions, peeled, halved, cut sides down, and one 2″ piece ginger, scrubbed, sliced lengthwise ¼” thick, and cook, undisturbed, until deep brown in spots, 5–7 minutes.
Step 2
Pour in 1 cup water, scraping up any browned bits with a wooden spoon, then add 3 lb. mixed soup beef bones (2″–3″ sections of oxtail, marrow, knuckles, and/or neck bone), rinsed under cold running water to remove blood and excess bits, 1 lb. beef brisket or chuck, cut into 2″ pieces, 2 Tbsp. fish sauce, ½ oz. yellow rock sugar or 3 tsp. granulated sugar, 2½ tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, and another 6 cups water (or just up to your max fill line). Lock lid and turn venting knob to sealing position. Cook on high pressure 1 hour (it will take about 20 minutes to come to pressure before the cook time begins). Naturally release pressure 30 minutes.
Step 3
While the broth is cooking, place 10 oz. dried thin Banh Pho rice noodles in a large bowl and pour in cold water to cover. Let soak 30 minutes. Drain noodles and rinse to remove excess starch.
Step 4
Freeze 8 oz. beef eye of round, sirloin steak, London broil, or tri-tip steak until firm, 20–30 minutes. Thinly slice against the grain, then chill in refrigerator until ready to serve.
Step 5
Once pressure on pot has released 30 minutes, place a kitchen towel loosely over vent to prevent splattering. Using a wooden spoon, gradually open venting knob. As soon as floating pin drops, carefully open lid. Using tongs, transfer bones to a large bowl.
Step 6
Place brisket in another large bowl and pour cool water over to cool down and keep from drying out. Using a fine-mesh strainer, scoop out solids from broth; discard aromatics and reserve any meat and bones for serving if desired. (Leftover meat and bones can also be reserved for another use; add to fried rice or bibimbap or scoop out marrow and slather it over slices of grilled toast.) Skim fat from surface with a ladle and discard. Taste broth and season with more fish sauce or salt if needed. Set to low sauté and simmer gently while you cook the noodles.
Do Ahead: Broth can be made 1 day ahead. Transfer to an airtight container; cover and chill. The fat will solidify on the surface, making it easier to skim and flavors will deepen. Strained and skimmed broth can be made 3 days ahead. Transfer to an airtight container; cover and chill, or freeze up to 3 months.
Step 7
Fill a large pot one-third of the way with water and bring to a rolling boil.