Pre-heat the oven to 200°C.
Cook the carrots in a large saucepan of salted boiling water for 3-4 minutes until just tender. Drain and leave to one side.
Whisk together the olive oil and Dijon mustard in a small mixing bowl with some seasoning. Brush the chicken breasts evenly with this mixture, then sprinkle generously and evenly with the mustard seeds. Arrange in a roasting tin, then spoon the carrots and garlic alongside them.
Roast for 20-25 minutes until the chicken breasts are golden and firm yet slightly springy to the touch. Cut into a thick portion and ensure it is cooked through, with no pink showing. Remove from the oven and spoon the chicken breasts, carrots and garlic cloves onto serving plates along with some of the juices from the roasting tin. Garnish with a sprig of sage leaves before serving.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.