Chop the broccoli stalks off of the florets, and trim a small slice of the base of the broccoli stalk off. Using a vegetable peeler, trim off the woody skin on the outside of the stalk and discard the outside. Shave the tender inside of the broccoli stem into long ribbons. Place ribbons in a large bowl. Peel the carrot into long ribbons in the same fashion, and place with the shaved broccoli in the large bowl.
Carefully slice the mushroom and red onion very thinly using a mandolin or a sharp knife, and toss into the bowl with the broccoli and carrots ribbons.
Roughly chop the cilantro and toss in the bowl.
For dressing:
Zest the lemon into a small bowl and set aside. Mix remaining dressing ingredients in a separate small bowl and drizzle onto the veggies in the bowl. Toss to combine. Spoon salad onto serving plates and garnish with lemon zest and chopped broccoli floret.
Notes
This can be served immediately, or prepared in advance; refrigerate, covered, for 4 to 6 hours.