Bangers and Mash With Onion Gravy

Name: Bangers and Mash With Onion Gravy
Prep time: 20 min
Cook time: 45 min
Total time: 1 h 5 min
Cuisine: British
Ingredients: 2 tablespoons vegetable oil; 2 tablespoons butter; 2 medium onions (peeled and thinly sliced); 1 teaspoon balsamic vinegar; 1 teaspoon sugar; 1 1/4 pints beef stock; 4 teaspoons cornstarch (or cornflour); 4 teaspoons water (cold); Salt (to taste); Freshly ground black pepper (to taste); 2 pounds potatoes (peeled and quartered); 6 tablespoons milk; 1 stick/110 grams butter (cubed); 8 thick sausages (pork, beef, or flavored)