Preheat the oven to 200°C (400°F/Gas Mark 6). Mix together the mozzarella, pesto and sundried tomatoes. Cut a slit into the side of each chicken breast and then stuff with the pesto – covered cheese. Wrap each stuffed chicken breast with 2 bacon rashers or slices of prosciutto di parma, not too tightly, but enough to hold the chicken together. Season with salt and pepper.
To make the rustic mixed potatoes, cut the potatoes and sweet potatoes into 5mm (1⁄4in) slices – a Japanese mandolin is perfect for doing this – and arrange in a single layer on a large roasting tray lined with parchment paper. Add the garlic and rosemary and season with salt. Drizzle over the oil and toss until evenly coated. Nestle the pesto-stuffed chicken breasts in the potatoes and roast for 20 – 25 minutes, until the chicken is cooked through and the bacon is crispy. Remove the chicken from the oven and rest in a warm place for 5 minutes. Return the potatoes to the oven in that time for a final crisp-up.
To serve, arrange the pesto-stuffed chicken breasts on warmed serving plates with steamed French green beans and the rustic mixed potatoes.