Preheat the oven to 350°F. Spray a 9 x 13-inch Pyrex baking dish with cooking spray.In a large saucepan, melt 4 tablespoons of butter over medium heat. Whisk in the flour. Reduce the heat to medium-low and cook for 1 minute, stirring once to avoid burning—it will bubble. Gradually whisk in the milk and 2 cups of water, then stir in the macaroni. Increase the heat to medium-high and bring the mixture to a boil. Stir frequently until the macaroni is just al dente, 8 to 10 minutes.Meanwhile, in a medium skillet, melt the remaining 3 tablespoons of butter over medium-low heat. Add the garlic and sourdough and toss to coat. Toast the crumbs until browned, stirring frequently to avoid burning, about 3 to 5 minutes. Remove the pan from the heat and let the bread cool slightly. Transfer to a food processor and pulse into crumbs. When the macaroni is al dente, remove the pan from the heat and stir in the cheddar, Havarti, Brie and cayenne. Season with salt and pepper. Stir until the cheeses have fully melted. Transfer the mixture to the prepared baking dish.Evenly sprinkle the toasted bread crumbs over the mac and cheese and place the baking dish on a baking sheet. Bake for 15 to 20 minutes, or until the crumbs are golden brown and the sauce is bubbling. Remove from the oven and garnish with fresh thyme. Enjoy!