Preheat the oven to 220C/Fan 200C/Gas 7. Roughly chop the sun-dried tomatoes. Heat the oil in a large frying pan and stir-fry the mushrooms over a high heat for 4 minutes.
Add the roughly chopped hazelnuts and fry for a further minute until the nuts are lightly toasted. Season with a good grind of black pepper and remove from the heat.
Stir in the tomatoes, garlic, breadcrumbs, parsley and chilli flakes until thoroughly combined. Break the cheese into small chunks and toss them through the stuffing lightly. Cut the peppers in half from top to bottom and carefully remove the seeds and membrane.
Place the peppers in a small foil-lined roasting tin, cut side up, and fill each half with the mushroom and feta stuffing. Cover the surface of the stuffing with a small piece of foil. Bake for 35 minutes until tender, removing the foil for the last 10 minutes of the cooking time. Serve warm with a lightly dressed mixed salad.