Joe Sevier at Epicurious says we should stockpile leftover citrus peels in the freezer:
“As I pulled a few slivers of frozen grapefruit peel from my freezer, I knew I’d made a smart move. I sliced the zest thinly lengthwise, chopped it across into a small mince, and scraped it all into a bubbling tomato sauce that had already been hit with harissa and white vermouth.”