Heat the oil in a large pan on low/medium heat. Add the sliced peppers and cook for 15-20 minutes, stirring regularly. You want the peppers to soften slowly and gently, so it helps to use a lid on the pan at this stage.
Once the peppers have started to soften, add the sliced onion and cook for a further 10 minutes.
At this point, add your pasta to a large pan of salted boiling water and cook according to package instructions.
Once the onions and peppers have softened, add the garlic, olives, and capers. Cook for a couple of minutes, stirring well.
Next, stir in the balsamic vinegar and allow it to cook off for a minute or so. At this point, start adding spoonfuls of the pasta cooking water to the sauce. The salty, starchy water helps to season and thicken it, and stops it from drying out. You can keep adding spoonfuls of the pasta cooking water and letting it cook away until you're ready to plate up. You want the sauce to be quite thick, but not too dry.
When the pasta is nearly done, add the chopped parsley to the stewed peppers and adjust the seasoning. Once the pasta is cooked, drain it and add to the pan with the sauce. Turn off the heat, mix well, and serve.