Preheat the griller. Beat together the butter and cream cheese in a medium bowl until well combined. Stir in the dill, chives, horseradish, lemon zest and lemon juice.
Grill the fish 15 cm from the heat for 2 minutes each side or until the fish is just cooked through. Serve topped with the herb butter and garnished with the dill sprigs. Arrange the salad leaves alongside.