Preheat oven to 325 degrees.
Lightly butter two mini-springform pans (4-inch diameter). Wrap the bottom and sides of each pan tightly in several layers of aluminum foil and set aside.
Combine the crushed crackers, 1 tablespoon of the Gruyere cheese and dill. Stir to combine then divide between the two prepared pans. Turn the pan to coat the sides and bottom evenly. Chill while preparing the filling.
In a small saucepan (with a tight lid) melt the 2 teaspoons butter on low heat. Add the minced white onion and cover with the lid. Cook, stirring occasionally for about 10 minutes. Set aside to cool.
Coarsely chop the smoked salmon and cut several thin strips for topping if desired. Set aside.
In a food processor pulse the cream cheese until smooth. Add the egg, remaining 3 tablespoons Gruyere, salt and sour cream. Process until smooth. Remove the mixture to a small mixing bowl. Gently stir in the cooked onion, sliced green onion and chopped salmon.
Divide the cream cheese mixture between the two prepared pans. Smooth the top and garnish with the thin strips of salmon if using.
Set the mini-springform pans in a larger roasting pan or casserole dish. Fill the pan with enough water to come halfway up the sides of the springform pans. Bake for 1 hour or until the center is set.
Turn off the oven and cool with the door ajar for 1 hour. Remove the springform pans to a rack to cool completely. Remove the foil and sides of the pans. Store in an airtight container until ready to serve. Bring to room temperature before serving. Garnish with fresh dill if desired.
To make the Horseradish-Lime Cream combine the sour cream, horseradish, salt, lime zest and lime juice. Blend well.