Combine eggs, cream, goat cheese, smoked salmon, scallions, dill, salt, and pepper in a bowl; set aside.
Heat oven to 350 degrees. Melt butter in a 10-inch nonstick skillet over medium heat. Add red onion; saute until translucent, about 5 minutes. Reduce heat to medium low. Pour reserved egg mixture over onions; cook, pulling back edges with a rubber spatula, until they begin to set, about 15 minutes. Bake in oven until set in center, about 20 minutes. Transfer from oven to a board. Let cool 5 minutes; run a spatula around sides and underneath the frittata, and ease out of pan onto the board. Cut into 12 wedges. Serve.