Char the ginger and onion under the broiler or in a dry cast iron skillet until deeply browned and blackened in spots, about 8 to 12 minutes.
Toast star anise, cinnamon, coriander, fennel, cloves, and black cardamom in a dry skillet over medium heat until fragrant, 1 to 2 minutes, then transfer to a spice bag or cheesecloth.
Blanch the chicken by covering with cold water, boiling for 5 minutes, then draining and rinsing the chicken and pot clean.
Return chicken to the clean pot with charred onions, ginger, spice bag, and 12 cups cold water. Bring to a boil, then reduce to a gentle simmer and cook 1 to 1.5 hours.
Remove chicken, let cool, then shred the meat, discarding skin and bones. Keep the broth simmering.
Strain broth through a fine mesh strainer, return to pot, and season with fish sauce, sugar, and salt. Keep warm over low heat.
Cook rice noodles in boiling water for 30 to 90 seconds until tender but chewy, then drain and divide among serving bowls.
Top noodles with shredded chicken, sliced onion, green onion, cilantro, and black pepper. Ladle boiling broth over each bowl and serve immediately with garnish plate.