I used sardines in this recipe but I really think mackerel would have been loads better, just because sardines have lots of tiny bones which are hard to make out under the topping.
I suspect white fish would also work well with this. I got the inspiration from a recipe in "Two Greedy Italians".
First of all gut, fillet and pin bone your fish and lay the fillets side-by-side in a well oiled baking tray. Cover with lots of breadcrumbs, chopped garlic, parsley, basil and a thinly sliced mild chilli or bell pepper.
Drizzle over some olive oil and season with salt and pepper. Bake in the oven at 180 degrees C, for about 25 minutes.
This was lovely with a watercress, tomato and cucumber salad.