A tart lemon vinaigrette complements the mushrooms and celery hearts perfectly.
Cut away peel and pith of lemon, and separate 3 segments from the white membranes. Thinly slice those 3 segments widthwise, and set aside. Into a small bowl, squeeze 1 tablespoon of lemon juice from remaining sections. Season with salt and pepper. Slowly whisk in olive oil until combined; set vinaigrette aside.
Divide greens between 2 salad plates.
In a medium bowl, toss mushrooms, celery heart, and all but several teaspoons of the vinaigrette. Arrange half the mushroom mixture in the center of each plate of greens; scatter chopped lemon over. Drizzle greens with remaining vinaigrette, and serve immediately.
This recipe calls for cremini mushrooms, an Italian cultivated variety; if you can't find them, use more white button mushrooms.