Prep aromatics: Smash lemongrass with the back of a knife to release oils. Slice ginger and crush garlic.
Simmer: Add water, lemongrass, ginger, garlic, lime leaves, and onion to a pot. Bring to a gentle simmer over medium heat.
Cook gently: Reduce heat to low and simmer uncovered for 25–30 minutes until fragrant.
Season: Stir in soy sauce (or fish sauce) and sugar if using. Taste and adjust.
Finish: Strain out solids. Stir in lime juice just before serving for brightness.
