Rinse or brush any dirt from the mushrooms and pat dry. Trim the stems and slice each mushroom into 1/4-inch thick slices.
Add the vinegar, water, garlic, salt, and sugar to a 4-quart saucepan. Bring to a simmer over medium-high heat, then stir in the mushrooms.
Cook the mushrooms for 2 minutes, then remove the pan from heat. Leaving the mushrooms in the marinade, let them cool to room temperature.
Once the mushrooms have cooled, drain them thoroughly, discarding the marinade. Add the shallot, olive oil, lemon juice, lemon zest, crushed red pepper, a few grinds of black pepper, and chopped parsley.
Combine thoroughly and set aside for 10 minutes before adjusting the seasoning to taste one last time.
Transfer the mushrooms to a serving dish, cover with plastic wrap and refrigerate for at least 2 hours before serving. Note: Marinated mushrooms will have the most flavor if made a day in advance.
Notes
Tips for Handling Mushrooms
Mushrooms are a rather delicate ingredient that require a little thought when handling. Here are some practical tips for the home cook:
Shopping for mushrooms: Don't buy fresh mushrooms too far in advance. Look for firm, unblemished mushrooms without dark or soggy spots. They should smell earthy, not fishy.
Storing mushrooms: Refrigerate mushrooms, unwashed, in a ventilated container and use them within a day or two.
Cleaning mushrooms: A quick rinse under cool running water should be all commercially grown mushrooms need to get them clean. Set them on a couple of thicknesses of paper towels and dry them individually right away. Don't leave fresh mushrooms sitting in a colander, and NEVER soak them.