In a large pot or Dutch oven, combine the broth, bruised lemongrass, sliced ginger, and smashed garlic. Bring the mixture to a gentle boil over medium-high heat.

Reduce the heat to low, cover the pot, and let the aromatics steep in the gently simmering broth for 10 minutes. This allows the fragrant essential oils to build a flavorful base.

Stir in the fish sauce and the halved baby bok choy. Simmer uncovered for 3 minutes until the bok choy stems just begin to tenderize and turn bright green.

Gently fold in the white fish chunks and fresh spinach leaves. Cook for 4 to 5 minutes, or until the fish is completely opaque and flakes easily with a fork. Be careful not to stir too vigorously, as the fish will break apart.

Remove the pot from the heat and stir in the fresh lime juice. Taste the broth and adjust the seasoning, adding an extra splash of fish sauce for saltiness or lime juice for acidity if desired.

Ladle the hot soup into deep bowls, ensuring you leave the tough lemongrass stalks and ginger slices behind in the pot. Garnish with fresh cilantro and sliced red chili, and serve immediately.