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1
Preheat the oven to 400u00b0F.
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2
Cut the onion into quarters, then cut the quarters crosswise and separate the layers to create wide, square slices. In a saute pan, heat 1 tablespoon of olive and add the onion. Season with salt and pepper, and saute over low heat until the onion has wilted and started to turn translucent; be careful not to let it brown. Add the red wine vinegar to the pan and shake it briefly, then transfer everything to the bottom of a medium-sized baking dish.
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3
Wipe the pan with a paper towel to clean it, then place it back over low heat and add the remaining tablespoon of olive oil. Add the anchovy paste and stir with a wooden spoon to dissolve the paste into the oil. Dump the breadcrumbs into the pan and stir to completely coat them with the oil. Transfer the breadcrumbs to a small bowl to cool slightly.
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4
Wash the herbs, pat them dry, and finely chop them. Add them to the bowl with breadcrumbs, and season everything with a small pinch of salt and a few grinds of black pepper.
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5
Lay the portioned fish on top of the onions and season with a bit more salt and pepper. Distribute the breadcrumb-herb mixture evenly on top of the fish. Add the white wine or water to the pan and scatter the olives around the dish.
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6
Bake the fish for 15 to 20 minutes, depending on the thickness of the fish; it should be cooked through but still moist; check the pan after 10 minutes and add a bit more water or wine if necessary.
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7
Serve immediately, with the some of the onions, olives and pan juices spooned alongside the fish.
