Simmer the potatoes in boiling water until tender. Remove from the heat, mash and set aside to cool.
Beat the mashed potatoes with the egg yolks, butter, 40g flour and the cheeses. Season well, cover and leave until cool enough to handle. Shape the mixture into 16 croquettes and place in the fridge to set for 30 minutes.
Take three separate bowls, place the rest of the flour in the first, the beaten eggs in the second bowl and the breadcrumbs in the last bowl.
Dip each croquette in the flour, pat off any excess, then coat in the egg before dipping in the breadcrumbs. Place on a tray lined with parchment paper and a coating of breadcrumb and place in the fridge until required.
In the meantime, prepare the gremolata: Combine the parsley, lemon, garlic and olive oil.
Season and toss to combine, then keep aside until ready to serve. Preheat enough oil in a shallow pan over medium/low heat or in a deep fat fryer.
Place the croquettes in the pan and cook for 2 to 4 minutes until coloured well all around. Remove from the pan and place on kitchen paper to absorb excess oil, season well and keep warm in the oven until required.
Serve the croquettes with a sprinkle of the gremolata.