1/4 cup red wine
2 tablespoons Dijon mustard
2 tablespoons Worcestershire sauce
1/3 cup brown sugar
1/2 cup butter
1 sweet onion, halved and sliced
1 pound fresh mushrooms, halved
salt and ground black pepper to taste
Stir the red wine, mustard, Worcestershire sauce, and brown sugar together in a bowl until the sugar is thoroughly incorporated into the mixture; set aside.
Melt the butter in a saucepan over medium heat; cook and stir the onion in the melted butter until translucent, 5 to 7 minutes. Add the mushrooms, season with black pepper, and continue cooking and stirring another 3 minutes. As the mushrooms reduce in size and begin to brown, stir the wine mixture into the mushroom mixture. Simmer until hot, 1 to 2 minutes. Serve immediately.