Rub the tomatoes, eggplants, and peppers with the oil and put them on a baking sheet, along with the onions. Roast in a 375°F oven for about 45 minutes, or until very soft (the onions may take as long as an hour). Allow to cool for 10 minutes, then remove the skin from the tomatoes and peel the onions. Cut the eggplants in half and scoop out the flesh. Roughly chop all the vegetables, then pass through a food mill into a bowl. Stir in the olive oil and season to taste with salt and pepper. Serve on pan tostado (toasted bread).