Place mushrooms in a large bowl and cover with boiling water to soak for 15-20 minutes. After fully re-hydrated, remove from water and squeeze to remove water.
In a large saucepan, add vinegar, olive oil, soy, sugar, garlic cloves, thyme and sliced onions.
Bring to a simmer to dissolve the sugar and then add mushrooms.
Remove from heat and allow to cool to room temperature for service.
You can also place in an airtight container and refrigerate for up to 3 months.