3 medium onions, thinly sliced
1 tablespoon all-purpose flour
1/2 cup red wine
1 (14.5 ounce) can chicken stock
2 teaspoons Dijon mustard
1 teaspoon Worcestershire sauce
salt and ground black pepper to taste
Melt butter in a heavy-bottomed pan over medium heat. Cook and stir onion in the melted butter until softened, about 10 minutes. Reduce heat to low. Cover the pan and cook until onions are caramelized, stirring occasionally, about 20 minutes. Stir flour into the caramelized onions and cook for 1 minute. Pour red wine, chicken stock, Dijon mustard, and Worcestershire sauce into the onion mixture; simmer until sauce is thickened, stirring frequently, about 15 minutes. Season with salt and black pepper. Typically poured over bangers & mash.