Preheat the oven to 350 degrees. Bring a large pot of water to a rolling boil.
In a large saucepan set over medium heat, melt 2 tablespoons of the butter. Add the sliced mushrooms, 1 tablespoon of thyme, and a pinch of salt. Cook, stirring often, until tender and deeply browned, about 10 minutes. A fair amount of liquid will be released from the mushrooms, as they cook the liquid should evaporate.
Transfer the mushrooms to a heat-safe bowl and set aside. Place the saucepan back on the heat.
To the saucepan, add the remaining 6 tablespoons of butter. Allow the butter to completely melt. Do not let the butter brown. Add the flour, whisking constantly as you do, then cook that mixture for 2-3 minutes or until toasted but still pale.
While the roux is forming, add the macaroni to the now boiling water. Cook until al dente, about 8-10 minutes. Drain.
Carefully pour the milk into the saucepan and whisk vigorously to combine. The roux will considerably thicken the milk. Cook for 5 minutes, constantly whisking.
Once thick enough to coat the back of the spoon, add in the condensed soup, 1 ½ cups mozzarella, 1 ½ cups sharp white cheddar, the seasonings, and the remaining 2 tablespoons of thyme. Whisk to combine.
To the cheese mixture, add the drained macaroni and sautéed mushrooms. Mix well.
Transfer the Mac and cheese into a 9x13 baking dish. Spread the remaining ½ cup of mozzarella and ½ cup white cheddar over the top. Bake uncovered for 35-40 minutes, or until golden brown and bubbly. Top with extra thyme and fresh cracked black pepper!