Lettuce Eat More Greens

I am a notoriously terrible lettuce grower.  I don't know if I just always plant at the wrong time, if my soil isn't right for it, or if I'm just not holding my mouth right.  My lettuce always bolts before it even gets big enough for me to be willing to pluck a leaf to see how it tastes.  Until this winter.  In late fall I planted some mesclun mix in my greenhouse, just a small patch as a test.  I didn't expect much given my track record, but in the past few weeks some of the lettuce has grown to testable size without any sign of bolting!

And I have discovered something because of my success: