I had a packed lunch for most of my working life, and vividly remember one of the old guys on site looking in at my "peace box" to see almost an entire plain loaf (mother's pride obvs) a hunk of battenburg and a variety of fruit and biscuits. He then looked up at me and gave me the single best piece of advice I've ever received, "Son, never leave your mother".
The fillings of choice from those days could be anything from cold mince to potted heed with loads of pepper. I'm actually drooling at the thought of it now.