In a microwave-safe bowl, heat the butter in a microwave oven for 1 to 1½ minutes, until the clear part of the butter separates from the milky solids. Reserve ¼ cup of the clear melted butter to coat the rillettes.

Pour the rest of the melted butter, including the milky part, into a small saucepan. Add the chopped onion and cook gently over medium heat for about 2 minutes, until the onion has softened. Add the fresh salmon and mix well, then reduce the heat and cook, covered, for about 2 minutes. Mix again. The salmon should be barely cooked in the center and still a little pink. Transfer to a plate.

With a fork, crush the poached salmon into a coarse mixture. Transfer to a bowl, add the chopped smoked salmon, salt, pepper, and horseradish, and mix well.

Divide the mixture among three small ½-cup molds; the molds should be about three-quarters full. Smooth the top of each one with the back of a spoon. Pour the reserved melted butter on top. Refrigerate until ready to use.

To serve, let the rillettes warm up at room temperature for about 1 hour, so the salmon and butter soften a little. Then stir the top layer of butter into the rillettes and serve.

The rillettes will keep for a couple of weeks in the refrigerator.