Preheat the oven to 200°C (400°F). Peel and very thinly slice the Floury potatoes (use a mandoline or a sharp knife) and place the slices in cold water to remove excess starch while you prepare the other ingredients.
Thinly slice the Yellow onions into half-moons and set aside.
Drain the Tinned pickled sprats (ansjovisfiléer), reserving a few tablespoons of the Brine from tinned pickled sprats to flavor the cream. Roughly chop the sprats (or leave them whole if you prefer distinct bites).
In a measuring jug, combine Heavy cream and Whole milk (about 2:1 ratio or to cover the potatoes once assembled), stir in 1–2 tablespoons of the reserved brine, and season with a light pinch of Fine salt and a grind of White pepper — taste carefully, as the sprats are salty.
Generously butter a medium baking dish with some Butter. Arrange a single even layer of potato slices on the bottom, sprinkle with some onions and a few pieces of sprat, then repeat layering (potatoes, onions, sprats) until the dish is nearly full, finishing with a top layer of potatoes.
Pour the cream and milk mixture over the layered potatoes so it comes about halfway up the sides. Press the layers gently so the liquid settles evenly; dot the top with a few small knobs of the remaining butter.
Cover the dish tightly with foil and bake for about 35–40 minutes, or until the potatoes are just tender when pierced with a knife.
Remove the foil, sprinkle the surface with Fine dry breadcrumbs and dot with any remaining butter. Return to the oven and bake uncovered for another 15–20 minutes, or until the top is bubbling and nicely golden.
Let the gratin rest for 10 minutes before serving to set slightly. Adjust seasoning with a little more White pepper or Fine salt if needed, and serve warm.
