Ultimate Cottage Pie

Name: Ultimate Cottage Pie
Prep time: 35 min
Cook time: 2 h
Total time: 2 h 35 min
Cuisine: British
Ingredients: ½ teaspoon baking soda; 1 tablespoon water; 2 pound Ground beef; ½ teaspoon salt; 1 tablespoon oil; 1 large onion (chopped); 3 celery stalks (diced); 2 carrots (chopped small); ¼ cup all-purpose flour; 2 cups beef stock; 1 tablespoon soy sauce; A good handful of fresh parsley (chopped); 8 ounces cremini mushrooms (sliced); salt and pepper (to taste); 1 tablespoon flour; ⅔ cup heavy cream; 2 garlic cloves (thinly sliced); ¼ teaspoon freshly grated nutmeg; Salt and freshly ground black pepper (to taste); 2 pounds russet potatoes (peeled and cut into 1/8-inch-thick slices); 4 ounces cheddar cheese (grated); ½ cup grated Parmesan