How to Make It
In a small bowl, whisk the mayonnaise with both mustards and the lemon juice; season with salt and pepper.
In a food processor or blender, pulse the pretzels until they resemble cornmeal. Transfer to a shallow bowl. In another shallow bowl, beat the egg whites until frothy.
In a large saucepan, heat 1 inch of vegetable oil to 350°. Set a rack over a rimmed baking sheet. Working in four batches, dip the squid in the egg; press out any excess egg. Dredge the squid in the pretzel crumbs and shake off any excess. Fry the squid in the hot oil until golden and crisp, about 2 minutes. Using a slotted spoon, transfer the squid to the rack. Season lightly with salt and pepper. Transfer the squid to a platter and serve right away, with the mustard dipping sauce.