A discrete lunch in winter is a poor idea. You have to stop, put on more insulation, eat (while your muscles cool down), and take off the insulation before getting going again. And then you start up hiking with cold muscles and a big lump in your stomach... Also hard to do in a cold windy location.
A much better approach is to nibble continuously. (ie lunch time is any and all time between breakfast and dinner.) Anything that can be eaten in small amounts works--gorp, nuts, chocolate bits, fig newtons, candy bits, snack bars, candy bars, cheese cubes... This should include a good mixture of carbohydrates (fast energy), protein, and fat (long-lasting energy). (More fat in colder weather.) Ideally, you carry your lunch food in your pocket so all you have to do is pause, grab something, shove it in your mouth, and continue. Otherwise, carry it and your water in some easily accessible location where you can get at them quickly for a quick bite and drink.
For things that freeze or become hard in the cold (eg cheese, chocolate), cut or break them up into small chunks at home and rewarm them in your mouth before biting down. (You can break a tooth on cold chocolate...)
I carry my gorp in a 250 or 500 cc wide-mouth bottle. Just take the cap off and take a swig. Easy to do with mittens on and can be done through some face masks.
A couple more relevant threads:
http://www.vftt.org/forums/showthread.php?t=19128
http://www.vftt.org/forums/showthread.php?t=26039
Doug