dispatch_async and calling a completion handler on the original queue

I've seen some related questions but none seem to answer this case. I want to write a method that will do some work in the background. I need this method to call a completion callback on the same thread / queue used for the original method call.

- (void)someMethod:(void (^)(BOOL result))completionHandler {
    dispatch_queue_t current_queue = // ???

    // some setup code here
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        BOOL ok = // some result

        // do some long running processing here

        dispatch_async(current_queue, ^{
            completionHandler(ok);
        });
    });

What magic incantation is needed here so the completion handler is called on the same queue or thread as the call to sameMethod? I don't want to assume the main thread. And of course dispatch_get_current_queue is not to be used.