In a medium bowl combine apple, celery, and pine nuts. In a small bowl whisk together oil, vinegar, sage, the 1/4 teaspoon salt, and 1/8 teaspoon pepper; toss with apple mixture. Cover and refrigerate up to 1 hour.
Melt the butter in a 12-inch skillet over medium-high heat. Sprinkle boned side of fish with salt and pepper; add 2 or 3 fillets to skillet; cook for 3 minutes. Turn and cook for 3 minutes more or until fish flakes easily with a fork. Transfer to serving platter and cover to keep warm. Repeat with remaining fillets, adding more butter, if necessary, and reducing heat slightly if fish begins to brown too quickly.