Toppings to serve: fresh bean sprouts, sliced spring onion, coriander, mint, red onion, lime wedges, Sriracha
Turn on oven to broil/grill at max setting. Place oven rack high inside the oven. Line a tray with foil and arrange onion and ginger in 1 layer. Broil for 20 to 30 minutes until charred in some spots.
Place steak in the freezer for 40 minutes to firm up. Slice thinly and set aside to bring to room temperature.
While that is happening, place cinnamon, star anise, coriander seeds, peppercorns and cloves in a dry frying pan and toast on medium heat, stirring frequently, for 3 minutes until fragrant.
Pour broth and water into a large saucepan and bring to a boil. Add the toasted aromatics, carrots, fish sauce, brown sugar, and salt. Add charred onions and ginger. Reduce heat and simmer uncovered for 40 minutes.
Using a mesh strainer or slotted spoon, remove and discard the solid ingredients from the broth. Season to taste. Turn the heat up and bring the broth to a rolling boil.
Cook noodles in boiling water for 2-3 minutes. Drain and divide the hot noodles into four bowls. Top with steak slices and ladle piping hot broth over the steak cooking it instantly. Serve with your choice of toppings and garnishes.