Make the cream cheese schmear. Using a hand mixer, beat the sour cream and cream cheese together until soft. Fold in the thinly sliced scallions and dehydrated onion and season with salt and pepper. Do this in advance so the flavors have time to meld.
Slice bagels in half, then place on a platter or in a basket.
Place cream cheese, seafood salads, capers, and cornichons in small bowls, then arrange on a large cutting board or serving platter.
Spread the salmon pieces, bacon, and pastrami on pieces of parchment or wax paper and arrange on the board.
Add the remaining ingredients around the board for easy access. Don't forget serving spoons, tongs, and spreaders for your guests! Dig in!