Prepare the salmon fillet: Place the salmon fillet in a steamer basket over a pot of gently simmering water. Add the butter and water to the bottom of the steamer so it creates extra steam as it cooks. Cover and steam for 8–10 minutes, until the fish reaches an internal temperature of about 145°F (63°C) — you'll see flakes form but it should still look moist inside.
Combine ingredients: While the salmon steams, place the chopped smoked salmon in a large mixing bowl along with the cream cheese and mayonnaise. Once the fillet is done, remove it from the steamer and let it cool just enough to handle — about 5 minutes.
Flake and mash: Using your hands or two forks, flake the warm fillet into the bowl (skin should come off easily). Add the minced garlic, white wine, lemon juice, horseradish, Dijon mustard, black pepper, and salt to the bowl. Use a fork or potato masher to combine everything until it's spreadable but still has some texture — you want chunks, not purée.
Chill: Transfer the rillettes to an airtight container and refrigerate for at least 1 hour before serving. This helps all the flavors meld together and gives you something more spreadable.
Serve: Spoon into a small bowl or dish and serve with sliced baguette, crackers, or crostini. Garnish with fresh dill and capers if desired. The rillettes will keep in the refrigerator for up to 4 days.
