Preheat your oven to 400 F / 200 C / Gas mark 6. Toss the potatoes and onions together and place in an oven-proof dish. Squeeze the lemon into a jug and mix with the oregano, garlic and 150 millilitres of freshly boiled water.
Pour over the potatoes and onions, season very generously and drizzle over the olive oil. Place in the oven and cook for thirty minutes. Carefully remove and gently stir to ensure the potatoes are coated in the liquid.
Sprinkle over the diced peppers and return to the oven for another thirty minutes or until the potatoes are golden brown and cooked. Remove from the oven and set aside to cool.
While still warm check the seasoning then transfer to a serving dish and top with crumbled feta and garnish with finely sliced spring onion or chopped flat parsley.