Pour oil into cast iron skillet over medium-high heat. Char both halves of the onion until dark brown to black. Remove the onion from the pan, set aside and brown the oxtails on both sides. Bring water to a gentle boil in a large stockpot. Add the onion, oxtails and remaining broth ingredients. Once boiling reduce to a low simmer and let cook for 3 to 4 hours, skimming off any foam that rises to the surface, tasting intermittently for seasoning. Once the broth has reduced by a quarter strain through a sieve or colander.
Heat Pho Broth in a large pot. While the broth is warming, bring 10 cups of water to a boil. Take off the heat and submerge the rice noodles. The noodles should be al dente in 5 to 7 minutes. Drain the noodles and set aside.
Rinse and dry the basil, cilantro and bean sprouts. Arrange on a plate with jalapeno slices and lime.
Divide the noodles equally into four bowls, top with sliced onions and scallions and ladle the broth over the noodles. Finally arrange the slices of flank steak on top each bowl. Due to the temperature of the broth and the thinness of the meat, the meat will cook through.
Serve soup with fresh herbs, jalapeños, lime and sriracha, hoisin sauce and fish sauce for garnish.
Use broth immediately or pour into shallow container and refrigerate.