Cut fennel bulb in half and thinly slice each half until completely shredded and dump into a big bowl. Remove outer layers of red onion and slice in half. Slice onion halves thinly and add to fennel slices. Thinly slice celery, starting at the bottom of the heart and working your way through to the tips of the stalks. Add to fennel and onions. Thinly slice mushrooms and herbs and add to the fennel.
In a separate bowl, whisk together EVOO, lemon juice and salt and pepper to taste. Add dressing to fennel mixture and toss. Serve.