Cut the onion and red bell pepper into rough chunks and put in a bowl; mince the jalapenos and add them to the onion and peppers. Drizzle about 1 tablespoon of the extra-virgin olive oil over the vegetables and toss. Add a good pinch of salt and some pepper, toss, and place on 1 side of a rimmed baking sheet.
Drizzle the peaches with another tablespoon of oil, being sure to coat the cut sides well. Put them, cut side down, on the other side of the baking sheet. Roast the fruit and vegetables until caramelized, about 25 minutes. Using tongs, pull the skin off the peach halves and discard.
Arrange 2 peach halves on each serving dish, cut side up, on a bed of the roasted vegetables.
Note: If freestone peaches are not available, split the peaches leaving the pits intact; they can be removed easily after roasting.