Step 1Combine the mayonnaise, mustard, sugar, sour cream and dill in a small bowl. (This can be made a day ahead and refrigerated, covered airtight. Serve chilled.)
Step 1Place the shallots, garlic, clam juice, water, vermouth, mustard seeds and bay leaf in a 2-quart saucepan. Bring to a boil, then reduce the heat and simmer the broth, covered, for 20 minutes. Pour the broth through a strainer lined with a double-thickness of cheesecloth. Cool the liquid. (This can be refrigerated up to 3 days or frozen.)
Step 2Arrange the fish in a single layer in a large nonstick skillet. Pour the liquid over the fish; it should be covered. If not, add water as needed to cover the fish.
Step 3Bring the liquid to a simmer over medium-high heat, then gently cook the salmon, uncovered, with the slightest simmer until the fish just begins to look opaque in the thickest part, about 10 minutes. Remove the pan from the heat. Cool the fish in the liquid 1 hour. (The fish will be cooked through and be very tender.) Use a spatula to transfer the fish to a platter. Reserve the liquid; it can be frozen for soups, sauces or poaching other fish. Refrigerate the salmon until chilled, at least 3 hours, wrapped airtight. You can also refrigerate it overnight, but it's best served the same day.
Step 4To serve, spread a thin layer of mustard sauce on each filet. Garnish the fish and platter with dill sprigs. Pass any remaining mustard sauce.