Split the peppers lengthwise in two, remove the white pulpy core with all its seeds, then use a vegetable peeler with a swiveling blade to skin the peppers. Cut the pepper flesh lengthwise into ½-inch-wide strips. You should have approximately 4cups.
Peel the potatoes, slice them very thin, wash in cold water, and pat dry with kitchen toweling.
Choose a rectangular ovenproof serving dish or pan that can contain the fish and all the vegetables. Put in 3tablespoons of olive oil; the peppers, potatoes, onions, garlic, and salt; and liberal grindings of black pepper. Toss several times to coat the vegetables well and place in the uppermost level of the preheated oven. Bake until all the vegetables feel tender when prodded with a fork, about 45 minutes, turning the vegetables over every 15 minutes.
Remove the pan from the oven and turn on the broiler.
Gather the vegetables in the center of the pan, making room at the sides for the fish fillets. Put in the fish; sprinkle with salt, black pepper, and bread crumbs; and trickle over it the remaining tablespoon of olive oil.
Slip the pan under the broiler. (Do not let the flame or heating element touch or even come close to the dish unless it’s flameproof.) After 8 minutes, turn the vegetables over. After another 7 to 8 minutes, the fish should be done. Before serving, draw some of the vegetables over the fish. Allow to settle for a few minutes before bringing to the table.