Preheat oven to 375 degrees. Oil a 7 by 11-inch (or 10-inch round) baking dish.
In a large skillet or sauté pan, heat olive oil over medium heat. Add celery, carrots, onion and garlic and cook, stirring occasionally, until vegetables are starting to soften.
Stir in mushrooms, ground lamb and ground beef. Sprinkle with kosher salt. Continue cooking, stirring occasionally, breaking up beef into small pieces, until beef is cooked through and no longer pink.
Stir in thyme and sage and cook for 2 more minutes.
Stir in beef stock, tomato paste, Worcestershire sauce, Maggi seasoning, cinnamon and allspice. Bring to a boil; reduce heat to medium-low and continue simmering for 5 minutes.
Sift flour over top; stir to combine and continue cooking for 3 minutes, or until it starts to thicken. Season to taste with salt and pepper. Adjust other seasonings (herbs, Maggi, cinnamon, allspice) to taste.
Spoon filling evenly into prepared baking dish. Place spoonfuls of prepared potatoes over top; smooth top with a spatula. If desired, drag a fork lightly through the potatoes to decorate.
Bake for 30 minutes or until filling is bubbly and potatoes are hot. If the potatoes aren't browned at all, switch the oven over to broil and continue cooking for 3 to 4 minutes, just until they start to brown.
If desired, garnish with chopped parsley and chives. Serve immediately.