Mocking functional interface call in java

I have a method implementation that calls a functional interface (Java 8) and I am trying to write a unit test for it. Here is the method I want to test:

// Impl
public Optional<ExternalRelease> getStagedRelease(AccessParams accessParams) throws DigitalIngestionException {
    String resourcesPath=getReleaseResourcesPath(accessParams).orElseThrow(() -> new ValidationException(WRONG_ACCESS_CONTEXT_EXCEPTION));
    String ddexFilePath=getReleaseDdexPath(accessParams).get();
    if( stageDataManager.directoryExists(resourcesPath) ) {
        List<Track> tracks = getTracks(accessParams).
                             orElse(new ArrayList<>());
        ExternalRelease externalRelease = null;
        //Verify if lite XML already exists
        if( stageDataManager.fileExists(ddexFilePath) ) {
            //Load externalRelease values
            String liteDdex = stageDataManager.loadFileContent(ddexFilePath).
                              orElseThrow(() -> new ProcessException("Lite DDEX content couldn't be read."));
            externalRelease = ddexManagerExecutor( (ddexManager) -> ddexManager.getExternalReleaseFromLiteDdex(liteDdex) ).
                              orElseThrow(() -> new ProcessException("External release couldn't be parsed from Lite DDEX."));
            externalRelease.setTracks(tracks);
            return Optional.ofNullable(externalRelease);
        } else {
            //Create lite ddex if it doesn't exist
            ExternalRelease releaseFromTracks=getReleaseFromTracks(tracks);
            String liteDdex = ddexManagerExecutor( (ddexManager) -> ddexManager.getLiteDdexFromAccessParamsAndExternalRelease(accessParams, releaseFromTracks)).
                              orElseThrow(() -> new ProcessException("Lite DDEX content couldn't be generated."));
            stageDataManager.writeStringInFile(ddexFilePath, liteDdex);
            return Optional.ofNullable(releaseFromTracks);
        }
    } else {
        return Optional.empty();
    }
}

Note the line within the second IF

 externalRelease = ddexManagerExecutor( (ddexManager) -> ddexManager.getExternalReleaseFromLiteDdex(liteDdex) ).orElseThrow(() -> new ProcessException("External release couldn't be parsed from Lite DDEX."));`

The ddexManagerExecutor is a functional interface using java 8 features:

public interface ddexManagerConsumer<R> {
    R process(DdexManager ddexManager) throws ProcessException;
}

private <R> R ddexManagerExecutor(ddexManagerConsumer<R> action) throws ProcessException {
    DdexManager ddexManager = null;
    try {
        ddexManager = (DdexManager) ddexManagerPool.getTarget();
        return action.process(ddexManager);
    } catch (Exception e) {
        throw new ProcessException("Error while accessing to ddexManager pool.");
    } finally {
        try {
            ddexManagerPool.releaseTarget(ddexManager);
        } catch (Exception e) {
            log.error("Error while releasing ddexManager instance.", e);
        }
    }
}

So in my unit tests, which I am trying to do the following,

@Test
public void getStagedReleaseTest_whenFileDoesNoExist() throws Exception{
    AccessParams accessParams = getDefaultAccessParams();
    File file = new File("src/test/resources/testTrack.mp3");
    String xml="<xml/>";
    accessParams.setFileName("testTrack.mp3");
    accessParams.setReleaseSlug("test-release-slug");
    Optional<List<File>> mockFileList = Optional.of(Arrays.asList(file));

    when(mockedStageDataManager.directoryExists(any())).thenReturn(true);
    when(mockedStageDataManager.getResources(anyString(), any())).thenReturn(mockFileList);
    when(mockedStageDataManager.fileExists(anyString())).thenReturn(false);
    when(mockedStageDataManager.loadFileContent(anyString())).thenReturn(Optional.of(xml));
    when(mockedDdexManager.getLiteDdexFromAccessParamsAndExternalRelease(any(), any())).thenReturn(Optional.of(xml));

    DigitalIngestionServiceImpl serviceSpy = spy(service);

    Optional<ExternalRelease> externalRelease = serviceSpy.getStagedRelease(accessParams);
    Assert.assertNotNull(externalRelease);


}

This breaks my unit test since I am not mocking the call for the private functional interface. I am simply accounting for mocking the following inner line:

ddexManager.getExternalReleaseFromLiteDdex(liteDdex)

But none for the functional interface call that calls the above method. Any clue how to achieve that ?