Heat cooking oil in a stock pot, swirling to coat evenly. Place the onion pieces into the stock pot and char for 4-5 minutes each side.
At the same time, place the beef bones in a separate pot and cover with water. Parboil the beef bones by bringing the water to a boil, until impurities float to the surface. Rinse the beef bones, then place them into the same stock pot as the charred onions.
Pour the 2 cartons of Good & Gather pho soup starter into the stock pot.
Add 3 tbsp of fish sauce, then bring the broth to a boil. Reduce heat to medium-low and simmer for 20-25 minutes.
While the broth is simmering, cook the noodles according to package instructions. Drain, then rinse briefly under cold water.
Slice the flank steak into thin slices.
Assemble each serving bowl by dividing the drained noodles into each bowl. Atop with sliced flank steak, then ladle the simmering hot soup directly over the steak to cook them. Submerge the steak into the soup to reach your desired doneness. Lastly, top the bowls with desired accompaniments and serve immediately. Enjoy!