Mix the flour with the eggs, milk, salt, pepper and lemon zest. Cover the batter and let it rest for at least 1 hour.
Then remove the pistils from all courgette flowers. Soften the crumb in the milk and blend in a mixer the capers along with anchovies, tuna, yolks, the squeezed crumb, parsley, salt and pepper. At this point stuff the courgette flowers with the mixture, then soak them in batter (with a little baking soda) and fry them in hot vegetable oil.
A good alternative is to add some mozzarella in the stuffing in order to obtain a more delicate taste. However, if you don't like mozzarella, you can replace it with scamorza, which also combines well with the anchovies, thus creating a unique plate.