On mandoline, set the blade at 1/16" and slice cucumber. Alternatively, you could use a large vegetable peeler.
Combine brown rice vinegar, mirin, lime juice, coconut palm sugar, and tamari. Set aside. The sugar will dissolve after awhile, but if you are in a hurry, you can put the mixture in the microwave and heat it for 10 seconds.
Chop serrano and mint, and add to the vinegar mixture. Add sliced cucumber.
Put in the fridge for at least an hour so ingredients combine. Garnish with chopped peanuts.