1. Heat the oil in a large skillet over medium heat.
  2. Add the onions and cook for about 3 minutes, until they start to soften.
  3. Toss in the peppers, sugar, herb sprigs, salt and pepper and stir until everything is well coated.
  4. Place a lid on pan and reduce the heat to low.
  5. Allow the vegetables to sweat, stirring occasionally, for about 30 minutes, until they are very soft.
  6. Take the lid off the pan and raise the heat to medium.
  7. Cook off the excess liquid, which will take about 3 more minutes.
  8. Stir in the balsamic vinegar and remove from the heat.
  9. Pluck out the rosemary and thyme twigs.