Preheat oven to 220 deg C (200 deg C fan-forced). Rinse and chop sweet potato into 1 cm chunks.
Place the sweet potato chunks and rosemary onto an oven tray lined with baking paper. Drizzle with olive oil, season with a pinch of salt and pepper and toss to coat. Bake on the top rack of oven for 25 to 30 minutes, until golden.
Heat a drizzle of olive oil in a medium fry pan over medium-high heat. Add the sliced leek and cook for 4 - 5 minutes, or until soft. Remove from the heat, crumble in the fetta cheese and add the chilli flakes (if using) and stir to combine. Season to taste with salt/pepper.
Place your hand flat on top of the chicken breast and cut through horizontally, without slicing all the way through. Open the chicken up like a book, rub the chicken with olive oil and season with a pinch of salt and pepper.
Transfer the opened up chicken to a second tray lined with baking paper and top with the leek and fetta mixture. Cook in the middle rack of the oven for 15 minutes, or until chicken is cooked through., and the leek is slightly crisp around the edges.
When the chicken has about 5 minutes of cooking time remaining, return the frying pan to a medium-high heat with a drizzle of olive oil. Add the green beans (with some garlic, if you like) and cook, tossing, for 5 minutes, until crisp but tender.
Divide the chicken, beans and sweet potato between two plates (and serve with a lemon wedge, if desired).