In a large pot of boiling salted water, cook mushrooms just until tender, about 3-4 minutes; drain well.

In a large bowl, combine mushrooms, olive oil, vinegar, red onion, garlic, sugar, oregano, peppercorns, red pepper flakes and bay leaf; season with salt and pepper to taste.

Transfer to an airtight container and place in the refrigerator for at least 8 hours and up to 5 days.

Serve at room temperature, garnished with parsley, if desired.