This works particularly well with a hearty frozen fish like haddock loins. Any kind of bacon or other cured meat may be used instead of the pancetta. If streaky bacon is used, you may want to cook it and crumble it before assembling the casserole.
Melt the butter. Using a brush, paint the bottom of a 2" deep rectangular baking dish with a coating of butter. The rest of the butter will be used later.
Using a cleaver or heavy knife, cut the frozen fish into large pieces. Scatter these around the baking dish.
Remove the dark green end, root end, and outer layer of the leek. Thinly slice the white and light green parts of the leek into disks. Wash these thoroughly to remove any grit, using your fingers to separate the disks into rings. Spread the leeks on top of the fish.
Sprinkle the herbs on top of the leeks. Fresh herbs may be used if on hand.
Cut the pancetta into 1/4" dice. Scatter across the leeks.
Thinly slice the potatoes and use them to cover the other ingredients by overlapping the slices.
Paint the remaining butter on top of the potatoes.
Sprinkle with salt and freshly ground pepper.
Bake in a 350 degree F oven for about an hour or until the potatoes are as done as you like.
Serve with a slotted spatula to leave excess liquid in the pan.