6 medium sized russet potatoes, peeled and thinly sliced to about ¼-inch thick (easiest done using a mandoline)
2 tablespoons butter
3 large yellow onions, sliced
1 teaspoon brown sugar
¼ cup butter
¼ cup all-purpose flour
1 cup chicken stock
1 cup heavy cream
1 cup mozzarella cheese
1 tablespoon fresh thyme, chopped
½ cup Parmesan cheese, shredded
½ cup Monterey Jack cheese, shredded
Salt, to taste
Freshly ground black pepper, to taste
Preheat oven to 350 degrees Fahrenheit.
In a large skillet, heat 2 tablespoons butter over medium-low heat until it melts. Add onions, season with salt and pepper, and add the brown sugar. Stir to coat the onions, and cook slowly, stirring often, for about 20 to 25 minutes, until the onions begin to caramelize.
Make the bechamel sauce. In a separate saucepan, melt the ¼ cup butter over medium heat, while the onions are cooking. When the butter has melted, add the all-purpose flour and whisk to fully combine. Cook for 2 minutes, whisking occasionally. Slowly whisk in the chicken stock, breaking up the clumps as you do. Slowly add the heavy cream and whisk to incorporate. Season with salt and pepper. Reduce heat to low and cook for about 10 minutes, until it thickens.
Remove the bechamel sauce from heat. Slowly stir in the mozzarella cheese in small batches, waiting until one batch has melted before adding more. Add the thyme. Taste and season with salt and pepper if needed.
Spread a small amount of the cheesy bechamel sauce along the bottom of a 9-by-13-inch casserole dish. Place a single layer of sliced potatoes in the sauce. Top with a single layer of the caramelized onions. Sprinkle with Monterey Jack cheese. Repeat layers, starting with the bechamel and followed by the potatoes, onions and Monterey Jack cheese. Continue until all ingredients are used, ending with the bechamel sauce.
Sprinkle the entire casserole with Parmesan cheese. Cover with aluminum foil. Place in the preheated oven and bake for 30 minutes. Remove foil and bake for another 30 minutes, or until the cheese has melted and the casserole is hot and bubbling throughout.