These mushrooms would be a good addition to an antipasto platter. For the most flavor, let them marinate overnight.
In a medium saucepan, bring water, olive oil, white-wine vinegar, garlic, thyme, salt, and ground pepper to a boil. Add mushrooms; stir to coat.
Cover; simmer over low heat until mushrooms are tender and have released their liquid, 5 to 8 minutes. Remove mushrooms with a slotted spoon to a bowl; reserve.
Return liquid to a boil; cook until reduced to 1/4 cup, 3 to 4 minutes. Pour over mushrooms. Refrigerate until completely chilled; serve.