Baltic garlic-herring casserole

This Is one of my favourite ways to prepare Baltic herring. In Germany, these small herring are known as Kronsardinen, and in Finland as silakka. When they are fresh and small, the bones are surprisingly easy to remove, leaving two beautiful little fillets
How to Make It
Take about 1 kg of fresh Baltic herring, preferably small-sized. Remove the heads, then gently pull out the backbone together with the attached bones. With small herring, the backbone usually comes away very easily, leaving two clean fillets.
Lightly butter a shallow ovenproof porcelain or ceramic dish, approximately 25 × 40 cm.
Arrange the first layer of herring fillets in the dish, skin side down. Scatter generously with medium-finely chopped or thinly sliced garlic, followed by freshly crushed white pepper and coarse sea salt. Then cover the fish generously with chopped fresh dill—the dill should almost completely cover the fillets. Finally, distribute a few small pieces of butter over the top.
Now add a second layer of fillets, this time with the skin side facing upwards. Repeat the seasoning in exactly the same way: garlic, white pepper, sea salt, plenty of fresh dill, and a few more small pieces of butter.
The Sauce
For the sauce, mix about 500 ml of full-fat cooking cream with approximately one teacup of homemade tomato pomodoro sauce, if you happen to have some ready. If not, a good-quality ready-made tomato pasta sauce works perfectly well. Alternatively, mix about ½ dl of good-quality ketchup into the cream. You can also use Italian panna da cucina, which thickens slightly as it cooks and gives the sauce a particularly nice, smooth consistency. Pour the cream and tomato mixture evenly over the fish. The liquid should almost cover the herring and the dill.
Preheat the oven to 180°C and bake for approximately 12–15 minutes, depending on the thickness of the fish layers.
In my experience, two layers of herring are ideal. Be careful not to overcook the fish. If it is left in the oven for too long, the sauce can become watery. Ideally, it should remain rich, smooth, and creamy.
Serving Suggestion
For me, the perfect accompaniment is either creamy mashed potatoes or, in true Finnish style, new potatoes cooked with plenty of fresh dill.
Dill-Cooked New Potatoes
Always place the potatoes into already boiling water. This helps them cook evenly without splitting or becoming watery.
Once they are just done, drain them immediately. Add plenty of freshly chopped dill, then cover the pot with a clean kitchen towel or a piece of kitchen paper before putting the lid back on. The towel absorbs the excess steam and helps keep the potatoes beautifully fluffy.
Tip: Stop cooking the potatoes while they are still slightly firm--al dente. They will continue cooking gently in their own steam while resting under the lid, and will be just right when served.