Jason.encode(%{
topic: "dummy",
msg: Jason.encode(%{a: "message blob"})
})
# or
~s'{"topic":"dummy","msg":#{Jason.encode(Jason.encode(%{a: "message blob"}))}}'
These variants should both work. I’m not sure about exact quoting here, so it might be, that you need to wrap the interpolated value in another pair of quotes.
I prefer the first approach for readability.