Char the onion halves and ginger directly over a gas flame, or under a hot broiler, for 5 to 6 minutes until blackened in spots. Rinse off any loose burnt bits.
Add the stock, water, charred onion and ginger, star anise, cinnamon and cloves to a large pot. Bring to a gentle simmer over medium heat.
Slide in the chicken breasts. Keep the broth at barely a simmer (small bubbles at the edge only) and poach for 15 to 18 minutes, until the thickest part reads 74C / 165F. Do not let it boil hard, or the broth turns cloudy and the chicken tightens.
Lift the chicken onto a plate and let it cool for 5 minutes. Keep the broth simmering with the lid off for another 15 minutes so the spices keep working. Shred the chicken with two forks.
Meanwhile, soak the rice noodles in hot tap water for 15 to 20 minutes (or follow the packet), until pliable. Drain, then dunk in a pot of boiling water for 15 to 30 seconds until just tender. Divide between two deep bowls.
Strain the broth through a sieve, discarding the solids. Season with fish sauce and sugar, taste, and adjust. It should taste slightly too salty on its own, because the noodles will dilute it.
Pile the shredded chicken over the noodles, add the scallions and the sliced raw onion, and ladle over the boiling-hot broth. Serve with bean sprouts, herbs, lime and chili on the side.