Return the pan to high heat and add sliced onion, carrots, and green beans.
Cook until onions and carrots soften and beans blister slightly.
Add ginger, garlic, and scallion whites and cook for 15–30 seconds.
Add fish sauce, soy sauce, and black pepper and stir another 15–30 seconds until very fragrant.
Add rice noodles and chicken stock, pressing noodles down to submerge.
Simmer 3–4 minutes until noodles soften, tossing gently. Noodles should be aromatic and savory, not saucy.
Turn off heat and fold in scallion greens.