1. Open the tins of butter beans, tip them into a sieve set over your sink and allow to drain for 3 minutes.
2. Place the beans in a food processor fitted with a blade and whizz to a fairly coarse, dry paste. If you don’t have a food processor, mash them in a bowl using a potato masher.
3. Tip the beans into a mixing bowl and add the drained tuna, mayonnaise, lemon zest, parsley and flour. Mix very well to combine, then season to taste with salt and black pepper.
4. Place the bowl in the fridge for 40 minutes so the mixture can firm up.
5. Spread some more flour on a plate and dust your hands with it. Roll the mixture into balls about the size of an apricot, then flatten to form neat round patties. Lightly dust top and bottom with the flour on the plate.
6. Heat 1 Tbsp vegetable oil in a non-stick pan and gently fry the patties for about 3 minutes on each side, or until crisp, golden and heated right through. Watch them closely, as they brown quickly. Drain the patties on a paper towel. (If you’re planning to serve them immediately, keep them hot in the oven.)
7. To make the dip, whisk together the mayonnaise, lemon juice and lemon zest, and season to taste with salt and black pepper.