Slice tuna as thin as possible and keep refrigerated until plating.
Roast the corn on an open flame, rotating frequently to ensure even coloring.
Slice kernels away from cob and add to a blender with the mayo. Pulse the blender several times until corn is well incorporated in the mayo. Remove and store until use.
Place halved limes face down on a preheated and lightly oiled pan. Cook until charred.
Spread corn mayo on tostada, add sliced tuna, salsa, and purslane leaves as garnish.
