Make the salsa and dressing first: toss all the salsa ingredients together in a bowl and set aside. Place all the dressing ingredients in a blender (or tall container if you have a hand-held immersion blender) and whiz until smooth and emulsified. It will be a creamy pale green.
Pat the scallops dry with a paper towel and heat a heavy skillet over medium-high heat with a drizzle of canola oil. Mix the mayo and wasabi paste and spread over each scallop; place the pistachios in a shallow dish and dip the scallops in to coat on both sides. Sear in the skillet for a minute or two per side, or just until cooked. Set aside on a plate.
Arrange the arugula on a platter and place the salsa and avocado over top. Set the cooked scallops on top, along with any rogue pistachios from the pan, and drizzle with dressing. Serve immediately.