Puree the avocado, mayonnaise, lemon juice, 1/4 cup water and 1/2 teaspoon salt in a blender. Cover and refrigerate until ready to use.
Pour enough oil to fill a large, heavy-bottomed saucepan, about a third of the way. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 375 degrees F.
Whisk together the cornstarch, flour and seltzer. Submerge the green beans in the batter. Working in batches, carefully drop the beans into the oil and fry until crispy and golden, about 3 minutes. Use a slotted spoon to transfer to a paper-towel-lined plate and sprinkle with salt. Serve with the avocado mayonnaise for dipping.
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