Char the onion and ginger by holding them with tongs over an open flame or placing under the broiler until blackened in spots, about 5-8 minutes. Let cool, then rinse and peel away charred skin.

In a dry skillet over medium heat, toast the star anise, cinnamon stick, cloves, and coriander seeds for 2-3 minutes until fragrant. Be careful not to burn them.

In a large pot, combine the charred onion and ginger, toasted spices, and broth. Bring to a boil, then reduce heat to low and simmer gently for 1.5-2 hours, skimming foam occasionally.

Strain the broth through a fine-mesh sieve, discarding solids. Return to pot and stir in fish sauce, sugar, and salt. Taste and adjust seasoning as needed. Keep hot.

Cook rice noodles according to package directions until just tender. Drain and rinse with cool water.

Warm serving bowls with hot water. Divide cooked noodles among bowls and top with raw beef slices and sliced onions.

Bring broth to a rolling boil and immediately ladle over noodles and beef (the hot broth will cook the thin beef slices). Serve immediately with herb and vegetable garnishes on the side.

Let diners customize their bowls with fresh herbs, bean sprouts, lime juice, and jalape?os to taste.