In a large bowl, add cucumber, tomato, lettuce, dill, onion, olive oil, toasted sesame oil, salt and pepper. The easiest is to add ingredients in order as listed and as you are preparing them like washing and chopping the vegetables.
Toss gently just until combined.
Serve immediately or within an hour.
Notes
To make ahead: Refrigerate all vegetables covered in a bowl, and add oils, salt and pepper right before serving.
Lettuce: Boston, red leaf, romaine, butter, green leaf lettuces are great.
Cucumbers: Any type of cucumbers (dill pickle, garden, long English, Persian), peeled or not, work.
Tomatoes: Any type of full grown, grape or cherry tomatoes work.
Toasted sesame oil: It gotta be toasted, find it in any grocery store or online. It is a staple in Asian cooking. Or just omit.