Peel and slice the potatoes as thinly as you can manage. (If you have a 'mandoline' or food processor with a slicing blade, it will make short work of this. But if you have decent knife skills, it should not take long.

Mix the potato slices with a generous amount of olive oil, salt, pepper and a few rosemary leaves that you will have finely chopped. If using the garlic, add it as well. (In the alternative, you can simply rub the baking dish with garlic, which gives a very subtle savoriness to the dish, suitable for more formal occasions.)

Lay the potato slices on the bottom of a baking dish large enough to accommodate the fish and potatoes comfortably. (If you want to be fancy, you can arrange the slices in neat rows or an attractive pattern for a more elegant look.)

Roast the potatoes in a moderate hot oven (190°C/375°F) for about 20-30 minutes. Then remove them from the oven and lay the whole fish, which you will have seasoned well with salt and pepper, over the potatoes, with the sprig of rosemary inserted inside it. If using, arrange the tomato halves around the fish. Drizzle the whole with some more olive oil and, if you like, a bit of white wine (not too much) which will help the potatoes to soften and add a slight tartness that complements the dish.

Roast the fish until done, usually about 20-30 minutes more. Let the dish rest and cool off for a good 5-10 minutes before serving.