How to Make It
Stir together lime juice, galangal, garlic, and 1/4 teaspoon salt in a medium bowl. Add fish, and toss gently to coat. Cover and chill until fish is opaque, about 20 minutes, stirring once after 10 minutes.
Strain fish, reserving marinade. Remove and discard galangal and garlic; set fish aside. Transfer marinade to a small saucepan; add 1 teaspoon sugar and remaining 3/4 teaspoon salt. Bring to a simmer over medium-low. Simmer, stirring often, until sugar is dissolved. Remove from heat. Taste mixture, and stir in up to 1 more teaspoon sugar, if desired. (Dressing should be bright, sweet, and salty. If the dressing tastes too acidic, add sugar to mellow.) Let dressing cool completely, about 20 minutes.
Meanwhile, toss together bell pepper, shallot, lemongrass, chile, peanuts, cilantro, and, if using, bean sprouts in a serving bowl. Add fish and dressing to bowl, and toss to coat. Season with salt to taste. Drizzle with 1 to 2 teaspoons olive oil, if desired, and serve with lettuce leaves.