Peel and slice the onions about ¼ inch thick. In a heavy bottomed pot over medium high heat. Melt the butter and add the onions. Cook the onions for 2-3 minutes or until they are slightly translucent. Add the salt and pepper and continue to cook the onions until they begin to caramelize. At this stage its import to stir the onions regularly so they do not burn. Once the onions take on an even dark brown color (about the color of milk chocolate and should take about 20 minutes) add the vegetable stock, Worcestershire sauce, and thyme, scraping the bottom of the pan to release any caramelized flavor. Reduce the heat and allow the sauce to simmer for about 5 minutes.
Stir in the diced potatoes and remove from the heat.
Arrange the cheese in the following order: take half the provolone slices and arrange them on the top in an even layer, follow with the shredded gruyere, grated parmesan, and another layer of the provolone (off set the provolone so the second layer fills any gaps from the first). Spray a piece of aluminum foil with nonstick spray. Place the foil (oiled side down) oven the casserole and place in the preheated 375 F (190 C) oven for 25 minutes. Remove the aluminum foil and bake for an additional 10-12 minutes or until the cheese is bubbly, potatoes are tender, and the top is golden brown.
Remove the casserole from the oven and garnish with the fresh thyme and flakey salt.
Enjoy right away, its ok to burn your mouth, it’s part of the fun and a crucial part of the experience.
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