Bring a large pot of lightly salted water to a boil. Add mushrooms and cook until softened, about 10 minutes. Drain and set aside to cool.
Combine olive oil, white wine vinegar, red onion, oregano, thyme, garlic, salt, and pepper in a large jar with a lid. Close jar and shake well. Add cooked mushrooms and refrigerate at least 5 hours. Remove from refrigerator 10 minutes prior to serving.
