Is there a way to test a RESTful api without actually running the backend computations in Serenity

Question

I'm attempting to test an api, but this api depends on running other services. Essentially the problem is that I want to test the api, but don't want to actually run those services, they are already tested elsewhere.

Here is a basic breakdown of what we are testing in general terms:

1. I have an api that makes a foo I want to test bar
2. Part of making foo makes the file bar
3. I have a lot of very different foo's and a lot of similar bar's
4. I don't want to make a new foo every time I want to check the bar

I've thought of but haven't come to a conclusion on:

1. Just test bar when I make a foo(but all the bar are in a similar service and probably shouldn't be tested separately)
2. Actually make a foo whenever I make a bar(time intensive, double testing, bad for Separation of Concerns)
3. Create a fake database of bar that the api tests call(requires db changes, not sure of effectiveness of this type of test)
4. Use some sort of Serenity/Cucumber magic

Other Issues:

1. When the tests are run, it's threaded, so I can't guarantee that foo occurs before bar.