Cut pork into 1/4-inch-thick slices; then cut slices into pieces about 2 inches long.
In a 4- to 5-quart nonstick pan over high heat, stir pork until most of the pink is gone, 2 to 3 minutes. Pour into a bowl.
Trim and discard root ends and coarse outer leaves from lemon grass. Crush lemon grass slightly.
Add lemon grass and broth to pan and bring to a boil over high heat. Reduce heat, cover, and simmer to blend flavors, about 15 minutes.
Meanwhile, rinse and drain mustard greens. Discard tough stems. Cut greens into 2- to 3-inch pieces.
Add mustard greens, noodles, and shrimp to broth mixture. Simmer just until noodles are tender to bite and shrimp are pink and opaque but still moist-looking in center of thickest part (cut to test), about 6 minutes. With a slotted spoon, lift out and discard lemon grass from broth. Return pork and juices to pan.
Break eggs into broth mixture and stir gently, breaking yolks, until eggs are set, about 1 minute.
Ladle soup into wide bowls. Add fish sauce, green onions, and red chili sauce to taste.