If I have access to a microwave, it's easy. I have a soft lunch bag that holds two plastic containers. I fill one with a leftover main course - curry and cauliflower rice or some other stew type thing, again over cauliflower rice (I eat low carb). The other container is salad. Chopped tomatoes, cucumbers and peppers go on the bottom and the greens on top. I tuck in a small plastic container of dressing and then close it up. After those two are in, I add an energy bar and some nuts for snacks as well as a small bottle of water that I've frozen the night before. Zips up neatly and goes in my panierre, or if I already have too much to carry, attaches by carabiner to my backpack.