Char the onion and ginger under the broiler for about 7 minutes, until slightly blackened.
Toast the cinnamon, star anise, cloves, and coriander seeds in a dry pan until fragrant, about 3 minutes.
In a large pot, add the beef stock, charred onion, ginger, toasted spices, fish sauce, and brown sugar. Bring to a simmer and cook for 2 hours.
Prepare rice noodles according to package instructions and set aside.
Strain the broth to remove solids, return to the pot, and keep warm.
Assemble the phở by placing noodles in bowls, topping with raw beef slices, and ladling hot broth over the top to cook the beef.
Serve with a plate of fresh herbs, bean sprouts, lime wedges, and sliced jalapeños. Offer hoisin and sriracha sauce on the side.