Sift the two flours into a large mixing bowl. Add the baking powder and water and mix to get a creamy consistency. Add the vinegar and put in the fridge for 30 minutes.
Meanwhile, for the turkey cake, mix together the soy sauce and ginger. Add the turkey slices and set aside to marinate.
Heat the oil in a deep-fat fryer to 175C/340F. Alternatively, heat the vegetable oil in a deep-sided, heavy-based saucepan until a breadcrumb sizzles and turns golden-brown when dropped into it. (Caution: Hot oil can be dangerous. Do not leave unattended.)
Sandwich each marinated turkey slice between two slices of potato, shaking off any excess marinade. Sprinkle the flour on a plate and dust each turkey ‘cake’ in the flour to coat. Dip in the batter, then carefully lower into the hot oil and fry for 6-8 minutes, or until the potato slices are tender and the batter is crisp and golden-brown. Set aside to drain on kitchen paper, keep warm.
For the peas, heat the butter in a frying pan over a medium heat. Add the shallot and fry for 3-4 minutes, or until softened but not coloured. Add the peas, mint, vinegar and a little cold water and stir well. Continue to cook for 2-3 minutes, or until heated through. Season, to taste, with salt and pepper.
Remove half of the pea mixture from the pan and transfer to a plate. Mash the remaining pea mixture. Return the reserved pea mixture to the frying pan, then season again if necessary.
Serve the turkey cakes with the crushed peas alongside.