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1
Scale, head, and gut each sardine. Lay the fish belly-side down and press firmly along the backbone with the thumb. Turn the fish over and, beginning at the tail end, pinch and lift the backbone free, working towards the head end; it should come away in a single piece. Run a finger along the cavity to locate and remove any remaining pin bones. The two fillets remain connected at the belly and at the tail. The fish is done when it lies open flat without tearing and the flesh shows no visible bone.
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2
Combine the breadcrumbs, parsley, basil, crushed garlic, grated Parmesan, beaten egg, and olive oil in a bowl. If using pine nuts, add them now. Mix until the ingredients are just bound — the stuffing must hold its shape when pressed but must not be wet or paste-like. A stuffing that is too moist will steam rather than crisp. It is ready when a small amount pressed in the palm holds a clean edge.
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3
Open each butterflied sardine flat, skin-side down. Place one tablespoon of stuffing on one fillet half and fold the other half over it, pressing gently to restore the fish's original shape. This is the step on which the dish lives or dies: the sardines must be arranged tightly in a single layer in a well-oiled gratin dish — packed close enough that they hold each other upright and cannot unfold during baking. Spaced apart, they will open, dry, and lose the juices the dish depends upon.
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4
Drizzle the sardines with olive oil and squeeze half the lemon over them. Bake at 200 °C for 15–18 minutes. The dish is done when the breadcrumb edges visible at the seam are golden and the skin is visibly crisp and lightly blistered; the juices pooled in the bottom of the dish will be fragrant with olive oil and the sardines' own rendered fat.
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5
Serve immediately from the gratin dish, garnished with lemon wedges. The accumulated juices in the dish are part of the dish; bring bread to the table to absorb them.