Preheat your oven to 375°F (190°C).
If you're using store-bought pastry, roll it out slightly to fit your tartlet tins. If you're making your own, roll out the pastry to about 1/8 inch thick and cut circles to fit the tins.
Gently press the pastry into the tartlet or muffin tins, ensuring there are no air pockets. Trim any excess pastry hanging over the edges.
In a mixing bowl, combine the smoked fish (flaked into small pieces), sweetcorn (drained), heavy cream, egg, horseradish sauce, and fresh dill. Season with salt and pepper to taste. Whisk until well combined.
Evenly distribute the filling into each pastry-lined tin, filling to just below the top.
Place the filled tartlets in the preheated oven and bake for 20-25 minutes, or until the pastry is golden and the filling is set.
Once baked, remove the tartlets from the oven and allow them to cool slightly before gently removing them from the tins. Serve warm or at room temperature, garnished with additional fresh dill if desired.