Place the halved onion and ginger under a broiler for 5-7 minutes until the surfaces are charred and blackened.
In a large pot, combine the beef broth, charred onion, charred ginger, star anise, and cinnamon stick.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to develop the aromatic flavors.
While the broth simmers, cook the rice noodles according to package instructions, drain, and set aside.
Slice the beef sirloin against the grain into very thin, almost translucent strips.
Remove the solids from the broth using a slotted spoon and stir in the fish sauce, sea salt, and black pepper.
Divide the cooked noodles into a large bowl and arrange the raw beef slices on top in a single layer.
Pour the boiling hot broth directly over the beef slices to cook them through instantly.
Top the bowl with bean sprouts, fresh Thai basil, a squeeze of lime, and a drizzle of sriracha before serving.