what's the best food for an upset stomach?

Not a food, but chewing gum - especially something minty.

Pepto Bismol.

7 Up

I agree with saltine crackers and applesauce, and dry toast is good, too. Ginger ale is also helpful. Many years ago, I worked with a woman who swore by coke syrup. https://m.media-amazon.com/images/I/31Tt5KSP0dL.jpg
I tried it and it did help. I was advised to stay away from dairy products for a day or two, because they're harder to digest. 🤷🏻‍♀️

Mulligatawny soup. Whenever I've thrown up, the contents look and smell like that variety of tinned broth anyway. I assume that's the currency of the upset stomach and the stomach knows what's best for it.

Besides all that, I fancy that it might be reminiscent of the smell of F. Murray Abraham's chin, the breast pocket of Walter Huston's shirt and Tilda Swinton's bra.

https://youtu.be/iVrBOJ8CAt0

Plain Rice Porridge

No food.