01 - Preheat the 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.
03 - In the same skillet, sauté the onion and celery until softened, about 5 minutes. Remove from heat and set aside.
04 - In a large bowl, combine the cubed bread, cooked sausage, sautéed onion and celery, and cranberries. Add the parsley, sage, 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. Stir in the shredded cheddar cheese and Parmesan cheese if using.
06 - Pour the egg custard mixture over the bread mixture. Gently toss until all the bread is evenly moistened. Transfer the mixture to the prepared baking dish and spread it out evenly. Let it sit for 10 minutes so the bread can absorb the custard.
07 - Bake uncovered for 45 to 50 minutes, or until the top is golden brown and the center is fully set.
08 - Allow the casserole to cool slightly before serving. Garnish with additional fresh parsley if desired and serve warm.