My kid found this in a spaghetti dish at chain restaurant.

I reassured her that it was a mushroom because that’s what it looked like at first. But when I tried to slice it in half to confirm, one side began shedding brown flakes while the other side proved indestructible. So then I began to doubt it. Please reassure me that this is some type of vegetable remnant. And be gentle with me if you think it’s obvious. Thanks!