Preheat oven to 375ºF. Butter a 3-quart baking dish; set aside.
Place sliced potatoes in a microwave-safe bowl, pour in water, wrap with cling film and transfer to the microwave; cook on “high” for 3 minutes.
Carefully remove the bowl, flip potato slices, cover again with plastic wrap and return to microwave; cook for another 3 minutes.
Arrange half the potato slices overlapping them to cover the bottom of the prepared baking dish; set aside.
Pat dry cod fillets and cut into 2-inch cubes. Season generously with salt and pepper; set aside.
Melt 2 tbsp. butter in a skillet over medium heat. When sizzling, add leeks, carrots and celery; season generously with salt and pepper. Sauté until they just start to soften, about 3 ½ to 4 minutes. Add pressed garlic and sauté for only 1 minute. Remove skillet from the heat.
To assemble… Spoon half vegetable mixture over potatoes. Place seasoned cod pieces on top and sprinkle on lemon zest. Add a thin layer of Cheddar cheese sauce; spread evenly. Add the remaining half vegetable mixture and top with remaining half potatoes overlapping to cover the surface. Sprinkle on breadcrumbs.
Spoon on the rest of cheese the sauce and sprinkle on hot paprika.
Transfer to the preheated oven and bake for 45 minutes or until bubbly and golden brown.