With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with a silicone mat or parchment paper.
On the prepared sheet, toss the bell peppers and onions with the oil. Season with salt and pepper. Roast for 20 minutes, then add the tomatoes. Continue roasting for 7 minutes or until the tomatoes soften. Sprinkle with the cilantro.
Delicious served with brown rice and lentil fritters (see recipe).