Halve the onion and slice as thinly and evenly as you can, no thicker than 1/8 inch. (A mandoline works well for this, but you can do it with a sharp knife and some patience.) Add the onions, mushroom slices and parsley to a large bowl.
In a small bowl, whisk the lemon juice and olive oil. Ladle the dressing over the salad and toss to combine. Season with salt and pepper.
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