You can, but fresh lemongrass can be laborious to use from fresh. It is very stringy and hard and there is a fair amount of waste involved. You can use fresh if you like. Cut away the meaty, whiteish bulb portion of the lemongrass, stripping away any hard outer and upper stalks. Mash the bulb with the broad side of a heavy cleaver, then mince as finely as possible. Measure out the amount called for in the ingredients.