Preheat oven to 200°C (fan forced).
Add potatoes and onion to a large roasting dish. Drizzle with 1½ tablespoons olive oil, season with salt and pepper and toss to coat. Spread out evenly and bake for 20 minutes.
Place chicken thighs in a bowl, add the melted butter, remaining olive oil, lemon juice and zest, garlic, mustard, paprika, Italian herbs and thyme. Season well with salt & pepper. Toss the chicken thighs to coat evenly.
Remove the tray from the oven. Sit chicken thighs on top of the potatoes and onions. Add any lemon garlic sauce left behind in the bowl.
Return to the oven and bake for 25 minutes, uncovered.
Scatter green beans over the dish, gently spooning some of the pan juices over them. Add lemon slices if using.
Bake for a further 15 minutes, until the chicken is golden and potatoes are tender.
Finish with chopped parsley and extra lemon juice to taste.
