I've started keeping those weird sad little warm bags of sweaty chopped onions that arrive with Indian takeaway for no reason and freezing them in a big bag of chopped onions so I have easy onions in the freezer.

Other things I'm freezing then using later in soups & stews to reduce food waste: broccoli stalks, the leaves around the head of cauliflower, kale stems, carrot tops.

If you have leftover butternut squash or pumpkin seeds, may I recommend repurposing Nigella's bar nuts recipe, which is a game changer. https://www.nigella.com/recipes/bar-nuts

My friend @SchoolVegan zests lemons and limes every time he juices them and adds the zest to a lil pot in the freezer, I have not progressed to this level yet.