boil the eggs and puree them as well (a fork should do it).
mix the potatoes and the eggs, add the grated cheese and the tuna, mix well.
lay your brik pastries one by one, put some of the mixture horizontally in the middle of the bottom half, fold the bottom over the mixture, fold the sides (you should now have a rectangular shape with an arc at the top) then just roll your brik (see fig2).
put some frying oil in a pan and delicately fry the briks in until they are done (see fig 3).
dispose in a plate and serve when a bit cool, you can cut the lemon in half and squeeze some juice over them.