what's the best food for an upset stomach?

Soft boiled egg over cubed toast.

Just cold water, no food.

I’ll skip food for a day and take plenty of bed rest. If ‘it’s coming out of both ends’ don’t add fuel to the fire. If things don’t settle down after a couple days see a Doctor, it’s likely serious by then.

Crackers &/or Ginger Ale almost always works for me.

Coca-Cola. Really! The carbonation can have a soothing effect, and cola actually has anti-emetic properties. I've seen doctors recommend it, but I'd actually doing it for years, just because I like coke and I'd found that it could put my cranky stomach in a better mood.

And when my upset stomach has settled down a bit, I generally have some yogurt. It's bland, soothing, and full of protein and probiotics.

Most indigestion is associated with hyper-acidity. Tums or pepto or baking soda (bicarbonate, which is a base) neutralizes the acid.

I'm thinking milk & bread. Like quasimodo's recipe ideas, but that takes a bit of tinkering in the kitchen.

Ginger ale

If you can't go without food, then the plainest of plain food, like toast without anything on it, rice, boiled egg whites, no yolk, etc.

Burned toast
Peppermint tea.