We tend to cook in quantities where there are leftovers: it's not really practical to cook exactly the right amount of food, and it's much better to have a bit extra than not enough. Plus with several people working from home we tend to go through a good amount of leftovers for lunches. One thing I've discovered recently with leftovers: thicker sauces and soups make surprisingly good spreads.

For example, I recently had tomato soup sandwiches:

While something watery or brothy will stay that way even when cooler, most things thicken up in the fridge to where they'll spread well on bread. And it's very tasty!

I still mostly eat any leftover soups and sauces reheated, but if I want something more portable I've been pretty happy with this.