1 whole lemon plus 1/4 c. fresh lemon juice
6 c. water
1 tsp. chopped fresh oregano
2 lbs. fresh medium mushrooms, ends trimmed
2 tbsp. white wine vinegar
1/2 c. extra-virgin olive oil
1 tsp. salt
3/4 tsp. fresh ground black pepper
2 tbsp. minced parsley
1 medium onion, minced

Cut lemon in half; remove seeds. Cut into very thin slices. In a large saucepan, bring water, lemon slices, and oregano to a boil. Add whole mushrooms, place a heatproof plate on top to keep mushrooms from floating to the top, and cook for 5-6 minutes, until mushrooms turn white. Drain.

Meanwhile, in a large bowl, combine lemon juice, vinegar, olive oil, salt, pepper, and parsley. Beat with fork to blend. Add hot mushrooms and onion to dressing and toss until well coated. Let cool, then cover with plastic wrap and marinate in the refrigerator, tossing occasionally, at least 8 hours or overnight. Serve slightly chilled or at room temperature.