Place the onions, ginger and garlic on a baking sheet and place under the broiler until charred on all sides. This can also be done on a grill or in a hot cast iron skillet.
Once charred, set aside and let cool.
Place all the whole spices (star anise, coriander, cloves, cinnamon, bay leaves) in cheesecloth or a small spice bag and secure it at the top.
Place the oxtails, charred vegetables and spice bag in a large 5 quart+ Dutch oven or stock pot.
Add the soy sauce, fish sauce, brown sugar and MSG. Pour in about 12 cups of water, leaving about 2" at the top to ensure it doesn't boil over. Adjust if you have a larger or small pot.
Bring the heat to boiling, cover, and reduce to low heat to simmer.
Simmer, covered, for 6 hours. Periodically lift the lid and skim off any foam that has risen to the top (this typically happens right at the beginning).
When it's done cooking, strain it using a mesh strainer. To get ultra clear broth, line the strainer with cheesecloth.
Reserve the oxtails for the soup.
Place the broth in a container and refrigerate 3 hours or overnight - long enough for the fat to rise and harden at the top.
Remove the fat from the top (this can be reserved for cooking other dishes in the future) and store in the fridge for up to 1 week, or freeze for up to 3 months).
Cook the rice noodles according to the package instructions and set aside.
Heat the broth to boiling. Taste and add additional salt if needed (I usually don't need to).
Divide the noodles into 4 bowls and add the slices of sirloin, pieces of reserved oxtails, bean sprouts and jalapenos.
Pour the boiling broth over top of the bowls and garnish with cilantro, basil, limes, hoisin and siracha sauce.
Serve hot with soups spoons and chopsticks.
Notes
At the beginning of the cook time, skim the broth periodically to remove the impurities.
Refrigerate the broth after cooking to allow the fat to harden at the top so it can be easily removed.
To get ultra clear broth, strain it through cheesecloth when initially straining, then again after removing the fat.
Substitute beef bones and brisket for the oxtails to get a similar style broth.