I suspect it is Lettuce, but can't be sure.
I grow one stem in water and another in soil to do a comparison.
Both are put indoor next to a window.
The one planted on water grow some new leaves, but the bottom stems might be rotting soon.
I transplant it to soil before it rot further in water. I use normal garden soil.
All the stems rotted.
I suspect that it is almost impossible to regrow vegetables in hot weather.