"{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}"
                                          ^^

That is where the error is, this matches exactly with what I said, the quotes aren’t escaped properly. The following should be valid:

~S'{"topic": "dummy_topic","msg":"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}"}'

Alternatively, you have to double escape in the msg field:

"{\"topic\": \"dummy_topic\", \"msg\": \"{\\\"fruit\\\":\\\"apple\\\",\\\"dish\\\":\\\"apple-pie\\\"}\"}"

I prefer the sigil approach for readability.

Due to the doublequotes in the value of msg beeing improperly escaped, you don’t have valid JSON.

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.

I thought you want msg to be a string containing JSON? Then of course you can not remove the wrapping quotes that make it a string.