While opinions vary, I think the best substitute for lemongrass is a combination of arugula, lemon zest and ginger as I think it comes closest to matching the complete flavor profile of lemongrass. The ginger and arugula capture the subtle herbal taste while the lemon zest captures the citrus flavor well. When using this combination as a substitute I would use 1/2 teaspoon of lemon zest, 1/4 teaspoon of ginger and one arugula leaf.