French Onion Potato au Gratin 9 Steps
Preheat the oven to 375°F.
In a small mixing bowl, combine the grated Gruyère, grated Emmental and grated Parmigiano-Reggiano and set aside.
Prepare the caramelized onions: Heat a medium-sized skillet on medium-low heat. Add in the olive oil and butter. Once the butter has melted, add in the sliced onions, kosher salt and cook for 15 minutes covered with a lid, stirring every 5 minutes or so. After 15 minutes, remove the lid and continue to cook for another 15-20 minutes stirring regularly until the onions have softened and are a deep golden colour. Stir in half of the chopped thyme leaves and fresh cracked black pepper, set aside.
While the onions are cooking, slice the peeled potatoes thinly, approximately 1/8 of an inch in thickness.
In a medium sauce pot, bring the 35% cream, finely minced garlic cloves, butter, kosher salt and fresh cracked black pepper to a gentle simmer. Remove from heat and set aside until ready to assemble.
To assemble the gratin: Layer half of the sliced potatoes in a casserole or baking dish, carefully spoon half of the warm cream mixture, then spread half of the caramelized onions evenly and sprinkle half of the cheese mix. Layer the remaining slicedpotatoes, cream mixture and then an even layer of the remaining caramelized onions. Cover tightly with tin foil and bake in the preheated oven for 30 minutes.
After 30 minutes remove the foil and allow to continue baking for an additional 30 minutes.
At this point remove from the oven and insert a knife into the centre of the potatoes, there should not be any difficulty inserting all the way through, (if there is, continue to bake for another 10 minutes or so). Turn the oven broiler to high, toss the remaining cheese mix with the panko breadcrumbs and olive oil. Sprinkle this mixture over the potatoes and broil until the entire surface is bubbly and golden brown, approximately 2-3 minutes.
Remove from the oven, top with remaining chopped thyme and allow to rest for 20 minutes before serving.