In a large pot over medium heat, combine all ingredients and bring just to a boil, then reduce heat to a simmer and cook uncovered until potatoes are just tender, 12 to 15 minutes. Remove potatoes from water and let cool completely.
Whisk together all ingredients in a bowl until there are no lumps.
In a blender, combine all ingredients and purée until smooth. Pass through a strainer to remove any larger pieces. Season to taste. Reserve refrigerated until use.
Heat a deep fryer to 350°F. Lightly dust potatoes with cornstarch, then coat them in Tempura Batter and drop them into the fryer one at a time, being careful they don’t stick together. Fry potatoes in batches until batter is golden brown and potatoes are hot in the center, 4 to 5 minutes. Season potatoes with salt and drain on paper towels.
Place some Sneaky Mayo on the bottom of a bowl or plate. Place 10 potatoes over mayo and add dollops of mayo on top of each potato. Garnish with scallions and any tempura bits that are left from frying.