Bangers with Onion Gravy

Heat oil in 12-inch skillet over medium heat until shimmering. Arrange sausages in pan and cook, turning once, until browned on 2 sides, about 5 minutes. Move sausages to 1 side of skillet. Add onions, evenly distributing around bottom of pan, and nestle sausages on top. Add ½ cup water and immediately cover. Cook, turning sausages once until they register between 160 and 165 degrees and onions have softened, about 10 minutes.

While sausages cook, whisk broth, Marmite, mustard, thyme, and rosemary in 4-cup liquid measuring cup until Marmite dissolves.

Transfer sausages to plate and tent with aluminum foil. Make sure onions are spread evenly; cook without stirring until beginning to brown, about 5 minutes (if onions have not browned, increase heat to medium-high). Continue to cook, stirring occasionally, until onions are tender and well browned, and dark fond forms on bottom of skillet, 3 to 5 minutes longer. Stir in sugar, pepper, and salt and cook for 1 minute.

Add broth mixture, increase heat to medium-high, and bring to boil. Cook, scraping up any browned bits from bottom and sides of skillet and stirring back into sauce, until sauce is slightly reduced, about 5 minutes. Combine cornstarch and remaining 1 tablespoon water in small bowl. Whisk cornstarch mixture into sauce and cook until sauce is glossy and has consistency of heavy cream, about 2 minutes.

Off heat, whisk in butter, 1 piece at a time. Stir in vinegar and season with salt and pepper to taste. Sprinkle with parsley and serve.