Butterfly pork tenderloins by splitting each horizontally in half, leaving 1/4 inch intact along other long side. Open up like a book and pound to flatten slightly. Set aside.
In skillet, melt butter over medium-high heat. Sauté onion, garlic and pine nuts for about 2 minutes or until onion is softened. Add mushrooms, cook for 3 to 5 minutes or until softened.
Transfer to bowl and add bread crumbs, parsley and sage. Season with salt and pepper to taste, mix well. Spread stuffing over tenderloins, leaving border all around. Fold each in half lengthwise and secure with greased skewers.
In a small bowl, whisk together oil and apple cider for basting. Cook tenderloins on greased grill over hot coals or on medium-high setting, turning and brushing occasionally with cider mixture, for 30 minutes or until pork is no longer pink inside.