If you associate salad crops solely with Summer, you really ought to explore growing chicories and radicchio. Many of them originate from the Northern parts of Italy and have evolved to be exceptionally hardy. Cold Winter conditions often cause them to droop temporarily, but when then temperature rises again, so do they! You might think this chicory is doomed to death, covered as it is with thick frost.
But No, it bounces back!
The effect of cold temperatures on the Radicchios is normally just to make their colour turn even darker red. They seldom even droop.
I also have several Endives left still. They are not so hardy, but will survive a sharp frost as long as it doesn't last too long.
Two days of continuous below-zero temperature would probably finish them off, but luckily that is not something we have had to contend with yet this Winter. We have had some frosty nights, that's for sure, but the temperature normally rises above zero during the day, allowing the plants to thaw out before the next frost.
This chicory is "Variegato di Castelfranco" (allegedly).
I say "allegedly" because it is not like the Variegato di Castelfranco I have had several times before. This one is small, with spiky leaves and very little heart, whereas the ones I have grown previously have been more the shape of a round lettuce, with big outer leaves protecting a creamy-white (but speckled) heart.
While we're on the subject of hardy salads, I'd also like to put in a word for Land-cress (aka American Cress).
I have grown it many times and found it to be very hardy indeed. Like the chicories, it flops when frosted, but always recovers. Definitely something you need to grow if you want Winter salads!