Bangers and Mash with Onion Gravy

Name: Bangers and Mash with Onion Gravy
Prep time: 5 min
Cook time: 25 min
Total time: 30 min
Cuisine: British
Ingredients: 4 pure pork sausages; 2 teaspoons olive oil; 1 cup water; 2 lbs. boiling potatoes (peeled or washed and unpeeled); Salt (for boiling water); 1 cup milk; 4 Tablespoons unsalted butter; 1/3 cup sour cream; Salt and freshly ground pepper; 1 medium onion (peeled and thinly sliced); 1 Tablespoon vegetable oil; 1 Tablespoon butter; 1 teaspoon white sugar; 1 teaspoon balsamic vinegar; 1 1/2 cups beef stock or broth; 1 Tablespoon cornstarch; 1 Tablespoon cold water; 2 teaspoons whole grain or Dijon mustard; Salt and black pepper