There are several elements you must prove in order to prevail in a negligence case. One of them is that you suffered some actual harm. It's not clear from your post that you've suffered any harm, beyond perhaps a histrionic episode in a public place.
Taking a piece of food home is also worthless evidence. At trial, they will defeat that evidence pretty quickly. Your photos and the testimony of eye-witnesses are your only worthwhile evidence. But again, if you have not suffered any harm from the incident, you don't have a negilgence case.
You may have other causes of action, but the value of those causes of action would probably be limited to your financial loss, which probably would have been mitigated had you allowed the manager to credit your bill for the cost of the unsatisfactory food.
It would cost you more to sue on these facts than you could recover if you won. I don't know if it will make you feel any better, but when it comes to foreign matter in foods prepared at restaurant, yours is a light-weight in the competition. A little internet research on this issue may leave you feeling like you got lucky.
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