Caramelize the Onions

1. Heat the butter and oil in a large skillet over medium heat. When the butter has melted, add the onions and a dash each of salt and pepper. Cook, stirring frequently, for 4 to 6 minutes or until the onions begin to brown in spots. Turn the heat to low, cover the skillet, and cook, stirring occasionally, for 15 minutes or until soft and golden brown. Add a splash of water or stock as needed.
   

   

Prepare the Gratin

1. Heat the oven to 400 F and butter an 11-inch gratin dish or 2-quart casserole.
   

   

2. Combine the cream, garlic, herbs, and a dash each of salt and pepper in a large saucepan and place it over medium heat. Heat just until it starts to boil. Remove from the heat, taste, and add salt and pepper as needed. Add the sliced potatoes to the cream mixture.
   

   

3. Arrange about 1/3 of the potato slices in a slightly overlapping layer in the gratin dish. Sprinkle generously with salt and pepper. Top with half of the caramelized onions and 1/3 of the Gruyère. Repeat with another 1/3 of the potatoes, salt and pepper, the remaining caramelized onion, and another 1/3 of Gruyère. Finish with a layer of the remaining potatoes and salt and pepper. Press the potatoes down and add the remaining cream mixture. Sprinkle with the remaining Gruyère and the Parmesan cheese.
   

   

4. Cover the gratin dish with foil and bake for 45 minutes. Remove the foil and bake for 20 minutes or until the top is golden brown and the potatoes are fork-tender.
   

   

5. Remove the gratin to a rack and let it stand for 10 minutes before serving.  Enjoy!