Preheat oven to 400F and grease a 9x13 baking pan.
Use a sharp knife to cut potatoes into thin slices 3/4 of the way down the potatoes into a hasselback shape. Place potatoes in baking pan and drizzle with olive oil and season with salt and pepper to taste. Roast potatoes at 400F 35-40 minutes or until potatoes are very tender. Remove from oven and keep warm.
In a medium skillet over medium heat, melt butter. Add onions to pan and saute, stirring occasionally until onions are a deep brown caramelized color, about 15-20 minutes. Add garlic, red wine, beef broth, parsley, and salt and pepper to taste. Cook onions 3-4 minutes until most of the liquid is evaporated.
Remove onions from heat and use a spoon to fill the insides of the hasselback potatoes with caramelized onions. Sprinkle swiss cheese over top.
Bake hasselback potatoes at 400F an additional 10 minutes until cheese is melted. Serve hot and enjoy!