For the fritto misto: In a medium bowl, mix together the flour, salt and pepper. In a large heavy-bottomed saucepan, pour enough oil to fill the pan about 1/3 of the way. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 375 degrees F. (If you don't have a thermometer a cube of bread will brown in about 1 minute.) Dredge the cauliflower, green beans, fennel, garbanzo beans, and lemon slices in the flour. Fry the dredged ingredients for 1 to 3 minutes until lightly browned. Drain on paper towels.
For the lemon mayonnaise: In a small bowl, whisk the mayonnaise and lemon juice together until smooth. Pour into a small dipping or serving bowl.
Arrange the fritto misto on a platter and serve with the lemon mayonnaise.