Place bread slices in a food processor and pulse into small crumbs. Transfer to a large tray or baking dish and mix in parmesan cheese and parsley. Season well with salt and pepper
Using a pastry brush, coat the chicken breast with the mustard. Press the chicken into the breadcrumb mixture, coating well on all sides.
Place a medium pan of salted water over high heat and bring to the boil. Add beans and cook until tender but still crisp, about 2 minutes. Drain and set aside
Heat the olive oil in large frying pan over medium-high heat until hot. Cook the chicken for 3 minutes on each side or until medium golden brown and cooked through, adding more oil as needed. Remove and place on a plate lined with paper towels and lightly covered with foil. Drain all except 1 tbsp of oil from the pan and return to the heat.
Reduce the heat to medium and cook the onion for about 2 minutes, until it starts to soften. Add the beans and cook a further 2-4 minutes until tender. Add capers and lemon juice and toss to combine. Season.
To serve, slice chicken and divide between 4 serving plates. Top with green beans and capers.