Heat the oven to 450F. While the oven heats up, spread the bones across a sheet tray. Have a quart of water at the ready.

Sprinkle the sugar evenly along the bottom of a large stockpot and place over medium heat. Cook, swirling the pan occasionally, until deeply caramelized, about 5 minutes. Add the star anise, cloves, and cinnamon, to toast in the caramel, swirling the pot until aromatic, 30 seconds. Quickly (and carefully!) add the water to halt the cooking and prevent the sugar from browning. Turn off the heat.

Roast the bones until deeply browned, about 1 hour. Transfer the bones and any fat that’s rendered from them to the stock pot. Add a splash of water to the hot tray and scrape up all the browned bits stuck to the pan, then pour it all into the stock pot. Add enough water to the pot to cover the bones by several inches and bring to a simmer.

Meanwhile, set a rack just under the broiler and heat to high. Place the onion and ginger, cut side up, onto the tray you just used to roast the bones. Broil until blackened, 8 to 12 minutes, then add to the pot with the bones.

Partially cover and very gently simmer everything together for at least 8 hours, but 12 is better if you have time. Periodically skim off and discard any scum that rises to the surface. Add more water if the level drops below the bones. Strain (you should have about 3 quarts—enough for 6 bowls of soup), then season with fish sauce and salt until it tastes like something you want to drink straight from the bowl.

To serve, cook the rice noodles according to the package instructions and divide them into bowls. Pile on whatever tender vegetables you like, thinly sliced or shaved with a peeler. Pour over the hot broth and garnish with torn cilantro, mint, and sliced jalapenos. Serve with lime wedges and sriracha alongside.