01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick spray and set aside.
02 - In a large skillet over medium heat, cook the sausage, breaking it into crumbles, until browned and fully cooked through, about 6 to 8 minutes. Transfer the sausage to a plate using a slotted spoon, leaving any drippings in the pan.
03 - In the same skillet with the residual drippings, sauté the onion and celery over medium heat until softened and fragrant, about 5 minutes. Remove from heat and set aside.
04 - In a large mixing bowl, combine the cubed bread, cooked sausage, sautéed onion and celery, and cranberries. Add the parsley, dried sage, dried thyme, kosher salt, black pepper, and red pepper flakes if using. Toss everything together until evenly distributed.
05 - In a separate bowl, whisk together the whole milk, heavy cream, and eggs until smooth and well combined. Stir in the shredded sharp cheddar cheese and Parmesan cheese if using.
06 - Pour the egg custard mixture over the bread mixture and gently toss until all the bread cubes are evenly moistened and coated.
07 - Transfer the mixture to the prepared baking dish and spread it into an even layer. Allow it to rest for 10 minutes so the bread absorbs the custard.
08 - Bake uncovered for 45 to 50 minutes, or until the top is golden brown and the center is fully set. A knife inserted into the middle should come out clean.
09 - Remove from the oven and allow the casserole to cool slightly for about 5 minutes. Garnish with additional fresh parsley if desired and serve warm.