01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish generously with butter or nonstick spray.
02 - In a large skillet over medium heat, cook the sausage, breaking it into small crumbles, until browned and fully cooked through, about 6 to 8 minutes. Transfer the sausage to a plate using a slotted spoon, leaving the rendered fat in the pan.
03 - In the same skillet with the residual drippings, sauté the onion and celery over medium heat until softened and translucent, 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, sage, thyme, 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. Stir in the shredded cheddar cheese and Parmesan if using.
06 - Pour the egg custard mixture over the bread mixture and gently toss until all the bread is evenly moistened and coated.
07 - Transfer the mixture to the prepared baking dish and spread it into an even layer. Let it rest for 10 minutes to allow the bread to fully absorb the custard.
08 - Bake uncovered for 45 to 50 minutes, or until the top is golden brown and the center is fully set and no longer jiggly.
09 - Remove from the oven and allow to cool slightly for 5 to 10 minutes. Garnish with additional fresh parsley if desired and serve warm.