I put spare food into plastic containers as I serve our meal and put it in the freezer immediately afterwards. The danger the experts perceive in putting hot food in a fridge or freezer is the danger that the fridge will be warmed above its recommended temperature.
But if you actually think about it. To do that and have the fridge above the recommended temperature for a significant period of time you would have to put a catering size dish at boiling point into a small fridge and most of the time we are talking about portions for one or two being put into, at the smallest, an under counter fridge or a bigger freezer.
Etheltbags Why not make a small casserole with only as much in it as you can eat? Buy a pound of, say, stewing beef, divide into 4 smaller portions and immediately freeze three of them and make a small casserole with the remaining 4oz. I often go the supermarket and buy just one carrot or one leek if that is all I need, that's the joy of loose vegetable displays. Again spare veg can be chopped and frozen raw if you have to much, ditto tinned veg if you do not need to use a full tin.
It does require planning menus and shopping but it doesn't just save food waste, it saves money.