Break lettuce coarsely into salad bowl; place tomatoes on top.
Sprinkle lettuce and tomatoes with salt and pepper.
Place onion slices in center of salad.
In a small frying pan over medium heat, toast sesame seed, stirring, until lightly brown.
Add butter and continue to cook over medium heat until mixture foams and lightly browns (do not let it burn).
Pour hot sesame-butter over salad; mix lightly.