In a large pan, melt butter over low-medium heat. Add onions and 1 tsp of salt. Cook, stirring occasionally, until the onions are caramelized and deep golden in color. 

While the onions are caramelizing, combine the cream, remaining salt, pepper, thyme, rosemary, and garlic in a small sauce pan. Bring the mixture to a simmer over medium heat, stirring occasionally. Once it’s reached a simmer, remove from heat and let stand for 15 minutes.

Preheat oven to 425℉. Peel and cut potatoes into ⅛ inch thick slices. Peel, core, and cut apples into ⅛ inch thick slices. Find a gratin pan, a deep dish pie pan will work great. Start with a thin layer of caramelized onion, then potato, and finally apple. Continue until you’ve used all of each ingredient. Pour the cream mixture slowly and evenly over the top.

Bake gratin in the preheated oven for about 40 minutes. The cream should be bubbly and the potatoes and apples should be tender. Sprinkle the parmesan on top and return to the oven. Increase the oven heat to a broil and cook another minute or two until golden on top.

Let sit for 10 minutes before serving. You can add extra thyme or rosemary to garnish. Serve and enjoy!