For onions: Slice onions into 1/8 inch julienned wedges. Wash the onions in cold water 2-3 times, allowing to soak for 10 minutes each time. Combine all brine ingredients until dissolved. Pickle overnight at minimum.
For sauce: Puree all ingredients thoroughly in a blender. Strain through a coarse strainer.
For ceviche: Toss diced tuna with scallions, olive oil and grey sea salt. Place directly into martini glass (or other plate ware) and drizzle with sauce. Top with pickled onions and plantain chips.