For each serving, grease a shallow 2-cup baking dish or gratin dish with /2 tsp of the butter or margarine.
In a small saucepan over medium-low heat, melt the remaining butter or margarine; stir in the bread crumbs and set aside.
Slice the potatoes and onion as thinly as possible and arrange them in alternating layers in the prepared dishes. Pour the cider or apple juice over the vegetables and sprinkle with salt and pepper. Bake for 20 minutes.
Reduce the oven temprature to 350 degrees.
Spoon the liquid in the baking dishes over the vegetables to baste them, then arrange the fish over the vegetables, folding the tips of any long, thin fillets under if necessary to fit them into the baking dish. Sprinke the buttered crumbs over the fish and bake for 10 to 15 minutes, or until the fish is opaque and firm to the touch and the crumb topping is golden brown.