Preheat the oven to 350F degrees Place a large pot onto medium heat. Add butter to pot and heat until frothy - add in the celery, onion, carrot and apple and garlic. Stir to coat with butter and season with kosher salt. Place an All-Clad grill pan on a separate burner over medium heat. Toss the mushrooms in a splash of the olive oil and grill until golden brown on all sides. Once the mushrooms are done, add them into the pot with the rest of the vegetables. Continue cooking the vegetables until they are soft - add sage, thyme, walnuts and sourdough. Check seasoning and stir well to combine everything while slowly drizzling in the beaten egg. Spoon the stuffing evenly into the All-Clad lasagna pan and bake for 1 hour. Serve on a trivet in the middle of the table with turkey or roast pork and all the fixings OR cover with the handy lid and take with you to a pot luck.