Transfer the peeled and deveined shrimp to a bowl and season with salt and pepper. Add the sesame oil and toss to coat. Transfer to the refrigerator until needed.
Start the broth:
Heat 1 tablespoon neutral oil in a large pot over medium heat. Add the potatoes and celery and cook, turning occasionally, for 5–7 minutes or until the celery softens. Season with salt.
Add the lemongrass paste and minced serrano pepper to the pot and cook for 1 minute until fragrant.
Simmer the broth:
Pour in the coconut milk and water and bring to a boil. Reduce heat to low and simmer for 20 minutes or until the potatoes are fork-tender. Taste and adjust the seasonings to your preference.
Cook the shrimp:
Once the potatoes are cooked, add the shrimp to the pot and simmer for 5–7 minutes or until opaque. Taste and season.
To serve:
Ladle the broth and shrimp into shallow bowls. Garnish with sesame seeds and a few drops of sesame oil. Arrange a few slices of serrano pepper in each bowl if you like. Enjoy!