To make the béchamel, melt the butter in a pot over medium heat. Add the flour, turn the heat to low and stir for approximately 3 to 4 minutes with a spoon until the flour no longer smells raw. Slowly drizzle in the cream while whisking, and cook on low heat, stirring constantly, for about 10 minutes until a simmer has been reached. Season with salt and pepper.
Bring béchamel up to a simmer in a large pot with the thyme. Add in the grated Gruyère and white Cheddar. Season with salt and pepper, and add cooked macaroni.
Once everything is mixed and warm, place into a baking dish. Top with bread crumbs and Parmesan, and bake in a 400° oven until hot in the middle, sides are bubbling and topping is golden brown.