How to create a non-blocking "slave" task?

I am using Simulink R2016a and I have a main model that is running at 200 Hz. I also have a function (call it function X) which takes variable time to execute, but in the range of 1-2 seconds. I would like to have the main model issue a call to function X, which will run function X in parallel. Once function X is finished, I want it to issue a call back to my main model to tell it that it is finished, at which point my model can use its results. All the while, I want to guarantee that my main model continues running at 200 Hz, regardless of how long function X takes to run.

How can this be achieved? An example Simulink model would be greatly appreciated. Thank you!