Set the oven to 350 degrees F. Prepare a 9x12-inch baking dish by spraying with nonstick pan spray or spreading with olive oil.  Add onion, garlic if used, celery, and carrots and sauté for 10 minutes over medium heat, stirring often. Add mushrooms, salt, pepper, and nutmeg, and continue to sauté for 5 minutes, stirring often. Stir in the chicken stock and bring to a boil. Remove the pan from the heat and let mixture cool slightly.

In a large bowl, combine onion mixture, including liquids, with eggs, potatoes, and matzos. Season with additional salt and pepper if desired. Turn the mixture into the prepared pan. Lightly sprinkle with paprika. Bake for 50 to 60 minutes or until browned. Remove from oven, garnish with chopped parsley or scallions, slice, and serve.