Preheat oven to 400°F (200°C).
Heat olive oil in a large oven-safe skillet over medium heat. Add the potatoes and season generously with salt and pepper. Cook for 20-30 minutes, stirring occasionally, until golden brown and tender.
While the potatoes are cooking, cut the chicken breasts into bite-sized pieces.
Once the potatoes are golden brown, push them to one side of the skillet and add the chicken pieces to the empty side. Season the chicken with salt and pepper.
Add the Brussels sprouts, chopped onion and lemon slices around the chicken and potatoes.
Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through and the Brussels sprouts are tender-crisp.
Garnish with fresh parsley (optional) and serve immediately.