1. Rinse and drain mushrooms; trim off and discard discolored stem ends. Cut mushrooms into quarters through caps.
2. 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.
3. Add mushrooms. Simmer, stirring occasionally, for 10 minutes. Let mixture cool, stirring occasionally. Serve cool or chilled.