How can I return a Flask redirect while wating for a request to complete?

from flask import redirect
import threading
import time, random

def quick_processing_function():
    return True

def long_processing_function():
    time.sleep(15)
    return random.rand() < 0.99

@app.route('target')
def function():
    result = quick_processing_function()
    if result:
        x = threading.Thread(target=long_processing_function_that_may_fail, args=())
        x.start()
        return redirect(SOME_EXTERNAL_SITE)