Cover the bottom of a capacious sauté pan with a generous coating of olive oil, then add your sliced fennel, a splash of water and sprinkle with salt. Give it all a good turn to coat the fennel slices, then cover and let the fennel simmer for about 15-20 minutes, or until the fennel has become very tender. While the fennel is simmering, uncover from time to time and give it all a very gentle turn—or, if you are feeling a bit adventurous, a flip, which will be less bruising to the fennel. Either way, however, by the end of the cooking, the fennel is likely to have broken up, which is perfectly fine.

Uncover the fennel and raise the heat to cook off all the excess liquid in the pan, turning (or flipping) the fennel frequently. If you like, you can let the fennel go a bit longer to caramelize it a bit. Season generously with salt and pepper. I like to add a handful of the fronds, finely minced, to the fennel just before removing it from the heat for some extra fennel flavor. Or you can add some fresh herbs if you like, like rosemary or marjoram.

Grease a gratin dish large enough to hold your fish fillets in a single layer (which, if you like, you will have rubbed with a split garlic clove) and into which you lay the fennel to make a 'bed' for your fish.

Season the fish fillets with salt and pepper and lightly oil them as well. Then lay the fillets in the gratin dish on top of the bed of fennel.

Roast the fish and fennel is a hot oven (200°C/400°F) for about 10-15 minutes, or until the fish is just done, taking care not to overcook the fish.

Serve the fillets on to plates, with a generous heap of fennel—which by now will have turned into a rough purée—on top of each fillet.