Peel the potatoes, cut them into 1-inch wedges, wash them in cold water, and pat dry with cloth towels.
Cut the peppers along their folds into lengthwise sections. Scrape away and discard the pulpy core with all the seeds. Skin the peppers, using a peeler with a swiveling blade.
If using round tomatoes, cut them into 6 to 8 wedge-shaped sections if using plum tomatoes, cut them lengthwise in two.
Peel the onion and cut each one into 4 wedge-shaped sections.
Wash all the vegetables, except for the onion and the already washed potatoes, in cold water, and drain well.
Put the potatoes and all the vegetables in the baking dish. They should not be too snugly packed or they will steep in their own vapors and become soggy. Add the oil, salt, and several grindings of pepper, and toss once or twice. Place the dish in the upper third of the preheated oven. Turn the vegetables over every 10 minutes or so. The dish is done when the potatoes become tender, in about 20 to 30 minutes. If after 20 minutes you find that the tomatoes have shed a lot of liquid, turn up the oven to 450°F or higher for the remaining cooking time. Do not be concerned if some of the vegetables become slightly charred at the edges. It is quite all right and even desirable.
When done, transfer the vegetables to a warm platter using a slotted spoon or spatula to drain them of oil. Scrape loose any bits stuck to the sides or bottom of the baking dish and add them to the platter, for they are the choice morsels. Serve at once.