Begin by halving the flank steak lengthwise, then place it in the freezer. This firming step, which takes about 30 minutes, allows for ultra-thin slicing that's essential for traditional pho texture.

While the beef chills, toast the whole spices. In a large Dutch oven or wide, heavy-bottomed pot over medium-high heat, add the cinnamon sticks, star anise pods, and black peppercorns. Stir frequently until they release their aroma and darken slightly—this takes about 3 minutes. Remove them from the pot and set aside.

Raise the heat to high and pour the canola oil into the same pot. Add the sliced white onions and ginger coins. Cook, stirring now and then, until they develop deep charred spots and caramelization, about 5 minutes. Carefully pour in the beef broth. Add the smashed garlic cloves and reserved toasted spices, stirring to combine. Let the broth come to a rolling boil, then reduce to a strong simmer and let it bubble away gently for 30 minutes to draw out maximum flavor.

Meanwhile, fill a large stockpot halfway with water, cover, and set it over high heat to boil. As the broth simmers, prepare the garnishes: arrange the red onion slices, jalapeño rounds, lime wedges, bean sprouts, and Thai basil on a small serving plate. Cover and refrigerate to keep them crisp and fresh.

Once the beef is slightly frozen and firm around the edges, slice it thinly—about 1/8 inch thick—cutting against the grain on a slight diagonal. Arrange the slices on a plate and refrigerate until it's time to serve.

When the broth is ready, strain it into a large bowl to remove all solids, then return the clear broth to the pot. Stir in the fish sauce and sugar, and bring it back to a gentle boil.

Add the fresh rice noodles to the pot of boiling water and stir briefly. Cook for about 5 seconds—just enough to loosen and slightly wilt the noodles—then drain immediately. If they begin sticking together, give them a quick rinse under hot water.

Quickly divide the noodles among four large bowls. Neatly layer the thin beef slices on top of the noodles in a single shingled layer. Ladle the steaming hot broth directly over the beef and noodles, allowing the heat of the broth to cook the meat just until tender.

Serve the pho immediately with the chilled plate of toppings, alongside hoisin sauce, Sriracha, and additional fish sauce for customized flavoring.