Hi Eyal,
The idea is to implement a custom FHIR operation as per the docs.
Once you set up the pre-installation subclassing 

(this is where you override those 3 classes:  
HS.FHIRServer.Storage.JsonAdvSQL.Interactions, HS.FHIRServer.Storage.JsonAdvSQL.InteractionsStrategy, HS.FHIRServer.Storage.JsonAdvSQL.RepoManager)

You can create a custom operations class (make sure you reference it in your Interactions class):

Class Testing.Internal.Interactions Extends HS.FHIRServer.Storage.JsonAdvSQL.Interactions
{

Parameter OperationHandlerClass As%String = "Testing.Internal.myOperations";

}

Class Testing.Internal.myOperations Extends HS.FHIRServer.Storage.JsonAdvSQL.BuiltInOperations
{

ClassMethod FHIRTypeOpCustompatienteverything(pService As HS.FHIRServer.API.Service, pRequest As HS.FHIRServer.API.Data.Request, pResponse As HS.FHIRServer.API.Data.Response)
{
        if pRequest.Type="Patient" { 
            if (pRequest.QueryString '= ""){                
                Set fhirService = ##class(HS.FHIRServer.Service).EnsureInstance("/api/internal/test1") //fhir server endpoint               Set request = ##class(HS.FHIRServer.API.Data.Request).%New()
                Set request.RequestPath = "/Patient"set request.QueryString = pRequest.QueryString
                Set request.RequestMethod = "GET"Do fhirService.DispatchRequest(request, .response)

                Set json = response.Json
                If json.%Get("entry") '="" {
                    Set iter = json.entry.%GetIterator()
                    While iter.%GetNext(.key, .value) { //for each patient entry request an everythingSet everythingRequest = ##class(HS.FHIRServer.API.Data.Request).%New()
                        Set everythingRequest.RequestPath = "/Patient/"_value.resource.id_"/$everything"Set everythingRequest.RequestMethod = "GET"Write !
                        Write"Patient ID: ", value.resource.id, !
                        Do fhirService.DispatchRequest(everythingRequest, .everythingResponse)
                        Write"Everything: ", !, everythingResponse.Json.%ToJSON()
                    }
                }
            }
            else{
                Set pResponse.Json = ##class(HS.FHIRServer.Util.Outcome).Create("information", "No Query String", "informational")
            }

        } 
        set pResponse.Status = 200
}

ClassMethod AddSupportedOperations(pMap As%DynamicObject)
{
	do##super(pMap) 
	Do pMap.%Set("patient-identifiers-everything", "http://hl7.org/fhir/OperationDefinition/patient-identifiers-everything")
}

}

Hope this helps.

Ari