Preheat 375 degrees F. Spray large skillet with cooking spray.
In a small bowl mix together 1 cup Gruyere, breadcrumbs, parsley and 3 to 4 tbsp olive oil. Set aside.
In a large bowl combine butter, garlic, thyme, remaining Gruyere, 1 tsp salt and 1/2 tsp black pepper. Fold in sweet potatoes until well coated. Arrange sweet potatoes with slight overlap in the skillet. Pour stock into bowl to collect any remaining bits and pour over potatoes.
Sprinkle Gruyere breadcrumb mixture in an even layer over the potatoes. Cover tightly with foil and bake for 30 minutes. Remove foil and bake until bubbling and golden brown, about 20 minutes. For deeper golden color, broil for last few minutes. Let it rest for 5 minutes before serving.