Tear lettuce into large pieces. Thinly slice onion and soak in water, then drain.

Mix canned tuna with mayonnaise, salt, and pepper in a bowl to make tuna mayo.

Sift flour into a bowl, add sugar and beaten egg, and mix. Gradually add milk while stirring, then strain through a sieve. Cover and refrigerate for 1 hour. Add melted butter and mix.

Heat a frying pan with vegetable oil, spread thinly using a paper towel, and pour half the batter. Tilt the pan to spread evenly.

Cook until edges start to brown, flip, and cook for 15 seconds. Repeat to make 2 crepes.