Pre-heat the oven to 190°C/170°C fan-forced. Arrange the onions, fennel, garlic cloves and grapes in the a large roasting tin. Drizzle over the oil, mix well and season with black pepper.
Top the vegetables with the chicken breasts and pour the wine into the roasting tray. Roast for 30 minutes, remove from the oven and add the walnuts. Roast for 25 minutes more, until the chicken is golden and cooked through.
Serve the chicken with the vegetables and grapes and dressing spooned over the top.