What are technologies that a time traveler could easily introduce to ancient times?

[Radar]

So, maybe I'm missing something, but is there something in a modern frier that could not be replicated with just a cauldron of boiling oil?

Surely deep frying is not a modern invention.

I think the only stop for proliferating deep fried food was the price of oil and how the process wastes large quantities of it for relatively little gain. No further than 100 years ago people commonly used even the water in which they cooked dumplings or vegetables to make some other dish as it still had some nutrients in it. Using up a highly caloric food just to deep fry something seems highly inefficient.