I had that same problem when I overwintered a few koi in my 55 gal. Just with hold food for a few days. They'll get hungry enough to investigate it. Also congrats on finding a place to dispose of your broccoli stems! "Sorry Mom, fish ate my food." lol I don't like the stems either.
Silly koi and goldfish won't eat the fancy koi food I got them because it floats. They don't realize it's there. -__- On the bright side, I found out they love chopped broccoli stems, which is great because I never eat the stem part, haha.