Melt the butter in a pan over a high heat, add the onions and sweat for 5-6 minutes until soft. Remove from the heat and add in the breadcrumbs, sundried tomatoes, parsley and mixed nuts, stirring to combine. Season with a generous amount of sea salt and ground black pepper.
Arrange the chicken breasts on a roasting tray and using a sharp knife, make a slit lengthways along the side of the chicken breast, but be sure not to cut the whole way through.
Stuff the inside of the chicken pocket with a heaped tablespoon of the stuffing mix.
Toss the pepper slices with a little balsamic vinegar and olive oil, and arrange around the chicken breasts.
Finally drizzle the chicken breasts with a little olive oil and season with sea salt and ground black pepper.
Place the roasting tray in the oven for 20-30 minutes until the chicken is golden and cooked through.
Serve the chicken with a few of the caramelised pepper slices on top and dig in.