Clean morel mushrooms.* In a large saucepan, cook morel or other mushrooms in boiling, salted water for 5 minutes; drain.
Meanwhile, in a small saucepan, combine the water, vinegar, black peppercorns, bay leaf, sugar, and salt. Bring to boiling. Reduce the heat and simmer, uncovered, for 5 minutes. Remove from the heat. Remove the bay leaf and peppercorns from the liquid mixture. Add the mustard seed.
Put mushrooms in a glass bowl. Pour liquid over all. Cover with clear plastic wrap. Marinate in the refrigerator for at least 6 hours. Makes about 1-1/2 cups.
Prepare the mushrooms as directed. Store, covered, in the refrigerator for up to 1 week.
To clean morel mushrooms, place mushrooms in a pan or bowl. Cover with water; add a dash of salt. Soak for 10 to 15 minutes. Drain, rinse, and repeat two more times.