Preheat the oven to 350 degrees F.

Saute the onion and butter in a 10-inch oven-proof omelet pan over medium-low heat until translucent, about 5 minutes.

In a large bowl, beat the eggs. Add the heavy cream, goat cheese, smoked salmon, scallions, dill, salt, and pepper and combine. Pour the mixture over the onions and place the omelet pan in the center of the oven. Bake the frittata for about 50 minutes, until it puffs and a knife inserted in the middle comes out clean. Serve hot directly from the pan.