The sentence is setting up an analogy, and what is a placeholder for the relationship of the food to the place:
A is to B as C is to D
A : B :: C : D
congee (the soupy rice porridge) : East/Southeast Asia :: oatmeal : Scotland
[The role congee plays] in the East/Southeast is what [i.e. the role oatmeal plays] in Scotland.
The horse-drawn cart was to the 19th century what the truck was to the
20th, a vehicle to haul goods and produce.
If the sentence above had stopped at "20th." then what would refer to an unspoken, implicit idea, which we could make concrete: "a vehicle to haul goods and produce". When the relationship is not made explicit, there is only a blank, what.
Congee, the soupy rice porridge is to East and Southeast Asia what oatmeal is to Scotland, a staple food.