Dredge the croquettes initially through the flour, shaking to remove the excess amount. Then immerse in the egg white, draining again the excess amount and finally dip in the breadcrumbs, making sure that the croquettes are covered very well on all sides. Repeat the process by passing the croquettes from the egg white and toast.
Pour enough sunflower oil (to reach the middle of the pot) in a saucepan large enough to fit the croquettes comfortably. Heat the sunflower oil to 180°C and add fry the croquettes until golden and bright and crisp. Remove and dry on absorbent paper.
For the mixture
Heat the broth and keep warm throughout the preparation.
Heat a saucepan (wide and shallow) over medium to high heat, add half the olive oil and sweat the onion, leek and spring onion. Season with salt and cook until soft.
Add the rice and sauté for about 2-3 minutes. Add the garlic, add the wine and let it reduce completely. Add the hot broth in small batches, stirring constantly and letting the rice absorb all the liquid before adding the next batch. Every addition of broth should cover almost all the rice. Cook in a gentle simmer.
Season with salt and continue to add broth, until the rice needs about 5 minutes of cooking and about 1/4 of the broth is left. Add the remaining olive oil, spinach and fennel and continue to cook in the same way, until the rice is cooked. Add the dill, fennel, spinach juice, lemon juice and salt. Allow the mixture to cool, then add the feta cheese. Shape into 50g balls and place in the fridge so that they thicken.