Melt the butter in a medium-sized saucepan over medium heat. Sauté the finely chopped onion and garlic until softened and fragrant.
Add the flour and cook, stirring constantly, until the mixture turns a light golden brown.
Pour in the brown beer, whisking continuously to ensure a smooth mixture.
Add the beef broth, mustard, brown sugar, salt, and pepper.
Bring to a simmer over medium heat and cook for 10-15 minutes, or until the sauce has thickened to your desired consistency.
Serve warm with roasted meats, mashed potatoes, or crusty bread.
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