Preheat the oven to 400 degrees. Place the peppers and tomatoes on a baking sheet, and roast until they are slightly charred, about 30 minutes.
Peel, and core the peppers and tomatoes. Transfer the flesh to the jar of a blender; set aside.
In a medium skillet, heat olive oil. Add onion, and cook until translucent, about 5 minutes. Add to the blender, and purée until smooth. Pass the mixture through a food mill fitted with a medium disk into a large bowl. Stir in the stock, orange juice, vinegar, salt, and pepper. Cover, and refrigerate until chilled. Serve garnished with yogurt and herbs.