Yes you can, but you need to slighlty cook the potatoes first. Parboil them or steam them or even microwave. Actually, microwaved won't be so nice, it may be rubbery and variable in how cooked they get, but point is
The sliced spuds on top won't get cooked at the right rate (same rate as the fish) if they aren't precooked or mostly precooked first.
You can, of course, just slice the raw potatoes on top, anyway. Cover the pie with foil and cook for longer. But the fish may well taste overdone (fish rarely needs cooking for long at all). And the potatoes will be crunchy from being cooked entirely in the oven.
The sliced potatoes on top will get a nice brown colour if you spread a little butter on them, just before popping in to the oven.
The sliced spuds on top won't get cooked at the right rate (same rate as the fish) if they aren't precooked or mostly precooked first.
You can, of course, just slice the raw potatoes on top, anyway. Cover the pie with foil and cook for longer. But the fish may well taste overdone (fish rarely needs cooking for long at all). And the potatoes will be crunchy from being cooked entirely in the oven.
The sliced potatoes on top will get a nice brown colour if you spread a little butter on them, just before popping in to the oven.
