In a medium sized bowl, whisk together the fish sauce, rice vinegar, sugar and juice from the lime. Whisk until the sugar has dissolved.
Add in the cilantro, chilies, garlic and ginger and whisk again. Add more rice vinegar if the sauce needs to thin out a bit more. Set this aside, it gets better as the ingredients meld together.
Deseed and thinly slice the melon - how you slice is up to you but I like thin long strips. Quarter the tomatoes and very thinly slice the cucumber. You want the cucumbers so thin you can almost see through them. Toss everything in the bowl with the dressing.
Use clean hands to toss the salad and combine.
To plate, scatter everything on a large plate in a thin layer, let the colors come through! Sprinkle with sesame seeds and dot with chili oil. Enjoy!