Prepare the rice noodles according to package instructions (usually boiling for 3-5 minutes), then drain, rinse with cold water, and set aside.

In a large pot, combine the chicken broth, lemongrass, galangal, and kaffir lime leaves. Bring to a boil over medium-high heat.

Reduce heat to medium-low and simmer for 5 minutes to infuse the broth with the aromatics.

Stir in the coconut milk, sugar, and fish sauce. Bring the mixture to a gentle simmer (do not let it boil vigorously to prevent separation).

Add the sliced chicken and mushrooms. Cook for 5-8 minutes until the chicken is cooked through and tender.

Remove the pot from the heat. Stir in the fresh lime juice immediately to preserve the fresh citrus flavor.

Divide the prepared rice noodles into serving bowls.

Ladle the hot soup and contents over the noodles and garnish generously with fresh cilantro before serving.