Prepare the filling: Boil the potatoes until just tender, about 5–6 minutes. Drain. In a skillet, fry the bacon until crisp, then add the onion and cook until soft. Stir in the potatoes, parsley, and season with salt and pepper.
Make the roux: In the same pan, melt the butter, whisk in the flour, and cook for 1 minute. Slowly add the milk, whisking until thickened. Combine this sauce with the bacon and potato mixture.
Assemble the pie: Preheat oven to 375°F (190°C). Line a pie dish with one pastry circle and spoon in the filling. Layer slices of hard-boiled egg on top. Arrange the sardines so that their bodies rest inside the filling while their heads poke out.
Add the crust: Cover with the second pastry circle, sealing edges around the fish heads. Brush pastry with egg wash if desired.
Bake: Bake for 30–35 minutes until the pastry is golden brown and crisp, and the sardines are cooked through.
Serve: Let rest a few minutes before slicing. Traditionally paired with peas or seasonal greens.