Heat 2½ tablespoons of the oil in a frying pan large enough to fit the onions comfortably. Drop in the onions with the bay leaf, thyme leaves and chilli and sauté very gently, stirring often – they must not colour at all. Cook for around 15 minutes, or until the onions are totally soft.
Drop in the garlic and cook for a further 2 minutes. Do not let the garlic burn.
Only when the onions are very tender and nothing less, add the vinegar and sultanas and keep cooking until the liquid has evaporated.
Stir in the sugar and cook until it has dissolved but not caramelized. Season with the sea salt and transfer to a bowl.
Preheat the grill to high and toast the slices of bread.
Divide the onion mixture between the slices of toast, brushing any leftover oil in the bowl over the sardines. Lay the sardine fillets on top of the onion mixture, skin-side up. Drip a little more oil over them, and then scatter over the Parmesan.
Place the toasts under a hot grill, not too close to the element, and grill for 4½–5 minutes, or until the sardines are brown.
Meanwhile, mix the remaining oil with the lemon juice.
Remove the sardines from the grill. Grind some black pepper and a little sea salt over the top of each fillet, then add the lemon zest and drizzle with the oil and lemon dressing.
To serve, scatter over a good amount of chopped marjoram or mint (marjoram is that much better) and the toasted pine nuts.