Bangers and Mash with Onion Gravy

INGREDIENTS

• 2 lb. good-quality bangers or pork sausages
• 1 Tbs. sunflower oil
• 2 lb. Yukon Gold potatoes
• Sea salt and freshly ground pepper, to taste
• 1/3 cup milk
• 2 Tbs. unsalted butter
• 1 Tbs. olive oil
• 2 red onions, halved and finely sliced
• 1 1/2 tsp. all-purpose flour
• 1 tsp. red wine vinegar
• 1 cup full-bodied red wine, such as Shiraz
• 1 cup chicken or beef stock