Rinse fillets in water; pat dry. Place oil or butter in skillet. Add shallots and saute 30 seconds. Add fish and saute 3 minutes on each side or until fish is flaky. Transfer fish to warm platter. Deglaze skillet with grapefruit juice. Add grapefruit sections and avocado slices to skillet to warm. Pour over fish just before serving. Serves 4 to 6.