How to Make It
In a medium bowl, combine the lemon juice with the vinegar. Whisk in the olive oil until well blended. Add the parsley, oregano, garlic and mushrooms, season with salt and pepper and toss to coat. Let stand at room temperature for 15 minutes. Discard the garlic clove.
Meanwhile, squeeze the lemon half into a medium bowl of water; add the lemon to the bowl. Working with 1 artichoke at a time, cut off and discard the stems. Pull off the dark outer leaves until you reach the tender yellow leaves. Using a serrated knife, cut 1 inch off the top of each artichoke, then slice very thinly and add to the lemon water.
Drain the artichokes and pat dry. Add the artichokes and celery to the mushrooms, season with salt and pepper and toss. Transfer to plates and top with cheese shavings.