You can use any of your favorite fillings, but since you'll be getting your greens in the form of the chard leaves, you can skip the lettuce and have more room for zesty diced veggies, hearty beans, ...
Mustard greens, garden sorrel, corn salad, kale, Swiss chard, arugula, and lettuce are all winter vegetable options. Different types of leafy greens have different temperature and sunlight ...