For the Vegetable fritto misto:
In a medium bowl, mix together the flour, salt and pepper. In a large heavy-based saucepan, pour enough oil to fill the pan about a third of the way.
Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 190°C. (If you don't have a thermometer a cube of bread will brown in about 1 minute.)
Dredge the cauliflower, green beans, fennel, chickpeas and lemon slices in the flour. Fry the dredged ingredients for one to three minutes until lightly browned. Drain on kitchen paper.
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.