Peel and thinly slice the onions. Sauté in a large casserole with the olive oil over low heat for 30 minutes. After 15 minutes, add the blond Smyrna grapes and pine nuts; continue cooking. Deglaze with sweet sherry and reduce for 2 more minutes.
Fillet the sardines, remove the central bones, rinse, and pat dry.
Two minutes before the onions finish cooking, add the sardine fillets, finely chopped herbs, salt, pepper, and Espelette pepper. Stir gently and remove from heat.
Preheat oven to 180°C (thermostat 6).
Layer 2 sheets of brick pastry and brush with melted butter. Spoon a quarter of the filling into the center, fold edges to form a rectangle, and place on a baking sheet. Repeat with remaining pastry.
Bake for 15 minutes.
Snip open the tops with scissors.
Serve hot with a seasonal salad. Pairs beautifully with Madiran wine.
