Fillet the mackerel, wash, dry and dip in flour which has been seasoned with salt and freshly ground pepper. Melt the butter in a pan large enough to take the fish in a single layer, and sauté the fish until golden on both sides. Meanwhile, chop the mushrooms and herbs finely and crush the garlic.
Remove the fish to a hot serving dish or 4 individual plates. Add the mushrooms and garlic