Clean mushrooms and lightly score the tops with a small knife.

Bring a pot of salted water to a boil and blanch mushrooms for 5 minutes.

Drain and let cool slightly.

Pack mushrooms into a clean jar with garlic, mustard seeds, red pepper flakes, peppercorns, and herbs.

In a saucepan, heat vinegar, water, sugar, and salt until fully dissolved.

Pour hot brine over mushrooms until fully submerged.

Drizzle olive oil on top, let cool, then seal and refrigerate.