1, For the broth
Char onion and ginger over an open flame (holding with tongs) or directly under a broiler until slightly blackened, about 5 minutes on each side. Rinse with water.
In a large pot, dry roast cinnamon, star anise, cloves, and coriander over medium-low heat, stirring to prevent burning. When spices are aromatic, add vegetable stock, soy sauce, carrots, and charred onion and ginger.
Bring broth to a boil, reduce heat, and simmer, covered, for 30 minutes. Strain and keep hot until ready to serve.
2, For the noodles
While broth is simmering, place noodles in a large bowl and cover with hot water. Let stand for 20-30 minutes or until tender but still chewy. Drain. (If soaking does not soften the noodles enough, blanch them in a pot of boiling water for a few seconds.)
3, For the toppings (optional)
While broth is simmering, prepare toppings as desired – slice and cook tofu, lightly steam or blanch vegetables, etc. Toppings should be unseasoned or only lightly seasoned so as not to interfere with the flavor of the broth.
4, To serve
Divide noodles between two bowls. Arrange toppings over noodles. Ladle about 2 cups of broth into each bowl. Serve with garnishes on the side, which diners should add to taste.