This has been an interesting discussion!
We have a gratitude reading we say before meals. Pulling back from the food waste issue, I think our family needs further discussion on gratitude and respect. I realize too, this isn't something they're going to get a grasp on immediately, but will be a process.
I do like introducing the idea that if we run out of an item, we're not getting more until the next month. I have been playing around with a) serving less food and b) buying less produce. Maybe this is new or maybe I wasn't paying attention before, but ds has mentioned a few times that he doesn't want to eat all that is on his plate as soon as it is in front of him. Even though it could mean more time in the kitchen, starting off with smaller portions will lead to less waste. I've been making more frequent trips to the market for produce instead of buying a lot of it at once. Also, being patient and explaining, calmly, as they are in the process of wasting - ie, I had washed strawberries on the counter last night. Ds took one, looked at it and announced he was throwing it away bc it had a green spot. Explaining that we can cut that off, that is a natural part of the plant etc.