1. Preheat the oven to 375 F (180 C). Quarter and slice the onion thinly. Dice all the vegetables into pieces that are approximately the same size (no larger than 1/2 inch). Pile the vegetables into a baking dish so that the vegetables lie a few inches thick. Season with salt, pepper and herbs to taste and then pour the olive oil over the vegetables. Mix thoroughly but gently - you don't want to destroy the tomatoes before the dish goes into the oven. Now check the vegetables to make sure they are well-coated and glistening with oil. If need be, add more oil.
2. Put the dish in the oven and cook for 45 minutes to an hour. Halfway through the cooking process, remove the dish from the oven and very gently stir the vegetables so that the ones at the bottom come to the top. Towards the end of the cooking process, stir a second time. Remove from the oven and let cool slightly. Check for seasoning and serve.