In the same skillet over medium heat, heat the oil and butter. Add the sliced onion and sprinkle with the sugar and 1/4 teaspoon of the salt. Cook, stirring frequently, until onions are softened and start to caramelize and brown, about 15-20 minutes.
1 Tablespoon oil, 2 Tablespoons salted butter, 1 large onion,, 1 teaspoon granulated sugar, 1/2 teaspoon salt,
Add garlic, and thyme, and continue to cook and stir for 1 minute. Whisk beef broth and cornstarch together, then add to the pan with the onions, along with the balsamic vinegar, scraping up any browned bits from the bottom of the pan.
2 cloves garlic,, 1/4 teaspoon dried thyme, 2 cups beef broth, 1 Tablespoon cornstarch, 2 teaspoons balsamic vinegar
Let the onion gravy come to a simmer, then reduce heat to medium-low. Simmer for 3-5 minutes, stirring frequently, until gravy has thickened, then taste and season with additional salt and pepper, as needed. Return the sausages to the pan with the gravy to make sure they are hot.
1/4 teaspoon black pepper
Serve the potatoes with the sausages, topped with a generous spoonful of the onion gravy. Sprinkle with chopped parsley and serve with green peas.
1 Tablespoon freshly chopped parsley leaves