Swedish Fish Gratin

Instructions

1. Prepare the ingredients
   Preheat the oven to 375°F (190°C). Butter a 9x13-inch (23x33cm) baking dish. Peel and thinly slice the potatoes, and thinly slice the onions. Cut the fish fillets into 2-inch (5cm) pieces.
2. Create the base layers
   Arrange half of the sliced potatoes in the bottom of the buttered baking dish. Sprinkle with salt and pepper. Add a layer of sliced onions on top of the potatoes.
3. Add the fish layer
   Place the fish pieces on top of the onion layer. Sprinkle with the lemon juice, half of the chopped dill, salt, and white pepper. Spread the Dijon mustard over the fish.
4. Complete the layers
   Add the remaining onions on top of the fish, followed by the rest of the sliced potatoes. Season the top potato layer with a little more salt and pepper.
5. Prepare the cream mixture
   In a small bowl, mix together the heavy cream and milk. Pour this mixture evenly over the casserole, ensuring it seeps down between the layers.
6. Add the topping
   Sprinkle the breadcrumbs evenly over the top. Cut the butter into small pieces and dot them over the breadcrumbs. This will create a golden, crispy crust.
7. Bake the gratin
   Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the top is golden brown and the potatoes are tender when pierced with a fork.
8. Serve
   Remove from the oven and let stand for 5 minutes. Sprinkle with the remaining fresh dill before serving. Serve hot with a simple green salad or steamed vegetables.