- Preheat oven to 400°F. Grease a 9x13-inch baking dish with cooking spray; set aside.
- Melt butter in a large pan over medium-high heat. Add onion, and sauté for 4-5 minutes until soft and translucent. Add garlic and sauté for an additional 1-2 minutes until fragrant. Stir in the flour until it is evenly combined; cook for 1 more minute. Gradually pour in the stock, and whisk until combined. Add in the milk, salt, pepper and 1 teaspoon of thyme, and whisk until combined. Continue cooking for an additional 1-2 minutes until the sauce just barely begins to simmer around the edges of the pan (avoid letting it come to a boil) and thickens. Remove from heat and set aside.
- Spread half of the sliced potatoes in an even layer on the bottom of the greased baking dish. Top evenly with half of the cream sauce, then sprinkle evenly with 1 cup of Freshly grated Boar's Head Sharp Wisconsin Cheddar Cheese and all of the Parmigiano Reggiano cheese. Top evenly with the remaining sliced potatoes, the other half of the cream sauce and the remaining cup of Wisconsin cheddar.
- Cover the dish with aluminum foil and bake for 30 minutes; the sauce should be nice and bubbly around the edges. Remove the foil and bake uncovered for 25-30 minutes, or until the potatoes are cooked through.
- Transfer the dish to a cooling rack and sprinkle with the remaining teaspoon of thyme and any extra Parmigiano Reggiano.
- Serve warm.