Start the broth: Set a large soup pot over medium-high heat and add the neutral oil.

Put the onion halves cut side down into the pot. Add the ginger slices in a single layer. Cook until both have dark brown spots, 4 to 5 minutes.

Turn the heat to medium. Add the star anise to the pot and stir for 30 seconds, just until fragrant.

Pour the vegetable broth into the pot. Add the soy sauce.

Raise the heat to high and bring the broth to a boil.

Lower the heat to a gentle simmer and cook for 10 minutes so the broth picks up the onion, ginger, and star anise flavor.

Cook the noodles: While the broth simmers, fill a medium saucepan with water and bring it to a boil over high heat.

Put the rice noodles into the boiling water. Cook according to the package until they are tender but not mushy, usually 3 to 4 minutes.

Drain the noodles in a colander. Rinse them briefly under cool water so they stop cooking, then let them drain well.

Set a sieve over a second pot or a large bowl. Pour the broth through the sieve and discard the onion, ginger, and star anise. Return the strained broth to the soup pot.

Finish the soup: Put the soup pot over medium heat. Add the sliced carrots and cook at a steady simmer until they start to soften, about 3 minutes.

Add the snow peas and the scallion whites to the pot. Simmer until the snow peas turn bright green and the carrots are tender, about 2 minutes.