Lettuce planted it the garden will usually not freeze until the dirt around the roots freeze. I plant just enough for us to eat for a few weeks and then plant another bed for later. I do the same with turnips.
I have had good luck with getting lettuce and spinach for most of the winter. I do plant some in pots indoors as well.
I just don’t like to buy lettuce and spinach at the store. You never know how it was grown or how it was handled. Even after I wash it I start thinking about the E-Coli outbreaks traced to fresh produce, and it just ruins my appetite. LOL.
Course if I don’t have my own, I will make do the best I can.LOL.
My wife uses her “salad wagon”. A wire wagon that hold 3 earth boxes that she plants with lettuce, collards and other salad fixins’. She positions it in the sun during the day and brings it into the heated garage at night. Here in Wisconsin we are still able to enjoy fresh greens through much of the winter.