Heat the oil in a frying pan, add the sausages, onion and juniper berries and cook over a medium heat for 10–12 minutes, stirring occasionally, until cooked through.
Pour in the red wine and stock, increase the heat and bring to the boil. Season well, then reduce the heat and simmer for about 5 minutes, stirring occasionally, until reduced by about a third. Blend the cornflour with the water, then add to the pan and stir briskly until the sauce is thickened.
Meanwhile, cook the potatoes in a saucepan of lightly salted boiling water for 8 minutes until tender, but not losing their shape. Drain, then return to the pan, toss with the cream and season well. Transfer to a shallow gratin dish and scatter with the Parmesan. Cook under a preheated medium-hot grill for about 8–10 minutes, or until bubbling. Serve with the sausages, scattered with parsley.