Grilled mackerel with escalivada & toasts

Escalivada is a punchy make-ahead Spanish dish of grilled peppers, aubergines and onions. It works really well with grilled or barbecued fish

3 very large or 4 medium peppers a mix of colours
1 red onion, halved and thinly sliced
3 tbsp extra virgin olive oil
2 medium aubergines
zest 1 lemon, juice of ½
1 rosemary sprig, finely chopped
2 tbsp small capers, drained
small pack flat-leaf parsley, roughly chopped
2 rosemary sprigs, finely chopped
3 garlic cloves, crushed
3 tbsp extra virgin olive oil, plus extra to serve (optional)
1 large olive ciabatta, cut into 8 slices
¼ tsp chilli flakes or hot paprika
4 mackerel fillets, pin-boned and cut in half if large (or 8 butterflied sardines)

Heat the grill as hot as it will go. Line the grill pan or a large baking tray with foil. Using a potato peeler, remove most of the skin from the peppers, then remove the seeds and slice into 1cm strips. Toss with the onion and 1 tbsp oil, then grill for 15 mins, stirring halfway, until soft and charring here and there.
Cut the aubergines into 1cm half moons and brush sparingly with 2 tbsp oil. Lay the slices over the peppers, season well, then grill for 5 mins until golden. Turn the aubergines over, scatter with the lemon zest and rosemary, then grill for 5 mins more until golden and soft in places. Stir the capers and the lemon juice into the vegetables. Season and set aside. Make and chill up to 3 days ahead; the flavours will intensify as it matures. Make sure you serve it just warm, with the parsley folded through it.
For the toasts and fish, mix the rosemary, crushed garlic, oil and some seasoning. Brush half of this over one side of the ciabatta slices. Mix the chilli into the remainder, then brush over the fish and let it marinate for anything from 5 mins to 1 hr in the fridge. Grill the fish, skin-side up (or barbecue skin-side down), for 4-5 mins, depending on the thickness of flesh, until just cooked through and the skin is crisp. Grill the bread until sizzling and golden. Top the toasts with the escalivada, followed by the fish, and serve with another drizzle oil, if you like.