Preheat and grease
Heat the oven to 200° C. Use a little of the Butter to grease a medium baking dish all the way up the sides and into the corners, so the casserole releases easily and the edges turn nicely crisp.
Soften the onions
Melt another knob of butter in a skillet over medium heat. Add the Yellow Onion and cook, stirring often, for 8-10 minutes until soft, glossy, and just starting to turn pale gold; keep the heat gentle so the onions sweeten instead of browning hard.
Prep the fillings
Peel the Floury Potatoes, then cut them into very thin matchsticks, about the thickness of a match, so they cook through evenly. Open the Tinned Pickled Sprats, lift out the fillets, and tear them into smaller pieces if they are long; reserve 2.5 tbsp of the Brine from Canned Pickled Sprats for the sauce.
Mix the sauce
In a jug or bowl, whisk together the Heavy Cream, Whole Milk, reserved brine, White Pepper, and a pinch of Fine Salt only if it needs it. The fish is already salty, so it is worth tasting the liquid before you add any more salt.
Layer the casserole
Arrange about half of the potato matchsticks in the greased dish, scatter over about half of the onions and sprats, then repeat with the rest, finishing with potatoes on top. Pour the cream mixture over the layers, scatter the Fine Dry Breadcrumbs evenly over the surface, and dot with the remaining butter. Press down very lightly with the back of a spoon so the liquid settles between the layers without packing them tight.
Bake until golden
Bake for 45-50 minutes until the potatoes are tender, the cream is bubbling at the edges, and the top is deep golden brown. If it starts to color too quickly before the potatoes are soft, lay a loose sheet of foil over the dish for the rest of the bake.
Rest and serve
Let the casserole stand for 10 minutes so it firms up a little, then spoon it onto plates or cut it into neat squares and serve warm.
