Step #1 Rinse & drain mushrooms; trim off & discard discolored stem ends.
Step #2 Cut mushrooms into quarters through caps.
Step #3 Meanwhile, bring 2/3 c water, vinegar, salt, dill seed, peppercorns, in a 3- to 4-quart pan over high heat, bay leaf, mustard seed, & garlic to a boil.
Step #4 Add mushrooms.
Step #5 Simmer, stirring every once in awhile, for 10 mins.
Step #6 Let mixture cool, stirring every once in awhile.