How to Make It
In a small saucepan, combine the water with the vinegar, sugar, kosher salt, mustard seeds, peppercorns, ginger, jalapeño and clove and bring to a boil. Simmer over low heat until the sugar and salt are dissolved.
Arrange the herring fillets in a large, shallow heatproof glass baking dish. Pour 1 1/2 cups of the hot pickling liquid over the fish; reserve the remaining liquid. Let the herring and pickling liquid cool to room temperature. Marinate the herring until it is almost opaque throughout, about 20 minutes, depending on the thickness of the fillets.
Meanwhile, in a small saucepan, sprinkle the gelatin over the fish stock and set aside until evenly moistened, about 10 minutes. Warm the gelatin mixture over low heat, stirring gently, just until warm to the touch.
Put the red onion slices in a shallow bowl and strain the pickling liquid from the herring over them; let marinate for 10 minutes. Pat the herring fillets and the red onion slices dry and wipe off any marinade solids.
In a large saucepan of boiling water, cook the asparagus until just tender, about 4 minutes; drain and refresh under cold running water. Cut the tips from the asparagus and set aside.
Line an 8-by-3 1/2-by-2 1/4-inch loaf pan with plastic wrap. Dip 10 asparagus spears in the gelatin mixture and lay them side by side on the bottom of the terrine. Layer one-third of the onions over the asparagus and spoon 2 tablespoons of the gelatin mixture on top. Dip 3 of the herring fillets in the gelatin mixture and arrange them over the onion slices in an even layer. Repeat the layering 2 more times. Cover the terrine with plastic wrap and refrigerate until firm, at least 2 hours or overnight.
Shortly before serving, strain all but 2 tablespoons of the reserved pickling liquid into a bowl. Add the asparagus tips and refrigerate for 20 minutes. In a small bowl, stir the crème fraîche, then fold in the caviar. Season with pepper and refrigerate. Strain the remaining 2 tablespoons of pickling liquid into a small bowl and whisk in the olive oil. Season the vinaigrette with table salt and pepper.
Unmold the terrine onto a work surface. Sawing gently through the plastic wrap with a serrated knife, cut the terrine into 1 -inch slices. Support each slice as you transfer it to a plate; discard the plastic wrap. Spoon the caviar cream on the slices. Garnish with the asparagus tips, chervil and dill, drizzle the vinaigrette over each slice and serve.