In a separate, shallow, nonstick pan, warm the oil and when it is hot, lower in a couple of the fish steaks. Let them colour lightly on the underside without cooking them right through, then turn and do the same on the other side. As each piece of fish becomes ready, transfer to the roasting tin, tucking them among the tomatoes and onions. Repeat with the remaining two pieces of fish. Once they are done, pour off any oil from the frying pan, return to the heat, and pour in the manzanilla, letting it bubble as you scrape at the sticky bits with a wooden spatula. Tip over the fish.