For the brine:
- 1/2 cup kosher salt
- 1 quart water
- 1 pound trout filets from small, 6 to 8 ounce whole fish, scaled, skin on, and cut into 16 to 20 (4 to 6-inches long by 1-inch wide) pieces
For the pickle:
- 2 cups water
- 2 cups cider vinegar
- 1 tablespoon sugar
- 8 whole cloves
- 8 whole allspice berries
- 6 whole black peppercorns
- 4 dried bay leaves
- 1 teaspoon red pepper flakes
- 16 to 20 cornichon and/or pickled onion
- 2 tablespoons Dijon mustard
- 1 medium onion, julienned
Place the salt and water into a 4-quart container and stir until the salt has dissolved, 1 to 2 minutes. Add the trout filets, making sure they are submerged. Cover and refrigerate overnight.
Combine water, vinegar, sugar, cloves, allspice, peppercorns, bay leaves, and red pepper flake in a 2-quart saucepan set over medium high heat. Cook, stirring occasionally, until the liquid comes to a boil, approximately 5 minutes. Cool to room temperature and then refrigerate overnight.
Remove the trout from the brine and rinse thoroughly under cold running water for 1 minute. Submerge the filets in clean cold water and refrigerate for 1 hour.
Drain and rinse the filets. Pat dry. Lay the filets in a single layer, skin side down and brush each with mustard. Place a cornichon or a pickled onion on the filet. Roll up each filet and secure with 1 or 2 toothpicks. Alternate layers of rollmops and julienned onion in a glass jar or ceramic crock.
Pour on the chilled pickling mixture, cover and refrigerate for at least 5 hours and up to 2 days. Drain and serve chilled with crusty bread.