Tumble the sausages, apples, onions and rosemary intoa roasting tin or shallow casserole dish (about 30cm across), where they can lie snugly in a single layer.
Mix together the maple syrup, mustard, olive oil and garlic, then pour into the pan. Season and turn the ingredients to coat.
Roast for 50 minutes, turning over the sausages once. They should be dark and glossy, with the apples completely soft.
Put the tin or dish on the hob and add the stock. Bring almost to the boil, stirring to help the maple and mustard mix with the stock. Add the blackberries to heat through and serve with mashed or baked potatoes.