- In a large pot of salted boiling water, cook the potatoes for about 20 minutes or until tender. Drain.
- With a potato masher, crush the potatoes with the butter. Add the cream and purée until smooth. Season with salt. Set aside.
- With the rack in the middle position, preheat the oven to 350°F (180°C).
- In a large non-stick skillet over high heat, brown half the fish cubes at a time in 2 tbsp of the butter. Season with salt and pepper. Set aside on a plate. Discard the cooking liquid.
- In the same skillet, soften the vegetables in the remaining butter (2 tbsp) for 5 minutes. Season with salt and pepper. Deglaze with the wine. Add the broth and cream. Let simmer over medium-high heat for about 10 minutes or until the sauce has reduced by three-quarters. Drain the fish pieces and add to the sauce.
- Spoon the fish mixture into four 1 ½-cup (375 ml) ramekins or into an 8-inch (20 cm) square baking dish. Spread the mashed potatoes over the fish. (At this point, the shepherd’s pie can be refrigerated or frozen.)
- Bake for 20 minutes (or 40 minutes if refrigerated or frozen). Finish under the broiler, if desired. Let cool for 10 minutes before serving.