Perhaps try something like this - For the base, I used a heavily subdivided (100x) plane and "grid circle" (as well as a subdivision Surface Modifier set to level 2). I added a solidify modifier as well, with Rim Fill unchecked so each piece is 2 (a poor way of compensating for lack of thickness) - Depending on your use needs, you may be able to get away with capturing it at the correct angle to mitigate this, otherwise you will have to give the mesh some real thickness. Either way, I suggest your starting plane be a bit more interesting than a square or circle (I lazily used them for the example).
As for the shader, I used a Musgrave texture as the basis for displacement - Musgrave goes all the way to -1, so I added 1 with a math node to bring it up to "base level". I also modified the subsurface radius RGB values to favor green (0.2, 0.8, 0.2). I also have it a small transmission value. Both the MixRGB node and the Displacement scale control the flatness of the "leaf" to slightly different effect.
I think with some combination of a "jagged" base plane, perhaps with some thickness and appropriately subdivided, and this texture, you can get pretty realistic looking burger lettuce (unless you need to be able to see the leaf "ribs" and "veins" - that's a different beast altogether).
EDIT - I also see lazy solutions are in order - if you want a really lazy solution, go here - https://www.cgtrader.com/3d-models/food/vegetable/lettuce-leaf-2a7276ae-ddf9-48e5-8c3a-364dcd2710cf - buy this model, and use it in your project.