Notes: If making ahead, cover and chill up to 1 week.
Rinse and drain mushrooms; trim off and discard discolored stem ends. Cut mushrooms into quarters through caps.
Meanwhile, in a 3- to 4-quart pan over high heat, bring 2/3 cup water, vinegar, bay leaf, peppercorns, mustard seed, dill seed, salt, and garlic to a boil.
Add mushrooms. Simmer, stirring occasionally, for 10 minutes. Let mixture cool, stirring occasionally. Serve cool or chilled.
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