2Wash and dry the peppers, then cut a lengthwise slit in each, leaving the stems attached. Remove the seeds and membranes carefully.
3In a bowl, combine the crumbled feta, spring onions, garlic, oregano, chili flakes (if using), 1/4 cup olive oil, red wine vinegar, and the handful of chopped fresh parsley. Stir gently until combined.
4Spoon the feta mixture into each pepper, distributing it evenly.
5Pour some of the remaining 1/3 cup olive oil into the bottom of a baking dish. Arrange the peppers snugly in the dish, slit-side up, and drizzle the rest of the oil over the tops.
6Bake until the peppers are tender and the filling is lightly golden.
7Let the peppers rest for 5–10 minutes before serving. Spoon the pan juices over them and finish with extra chopped parsley.