Melt unsalted butter in a skillet over medium heat. Add shallot, red onion, garlic, and mushrooms; cook and stir until mushrooms begin to give up their liquid, about 10 minutes.
Pour beer into skillet; simmer until liquid is reduced by half, about 10 minutes. Add rosemary sprigs and stir in Cajun seasoning. Simmer another 5 minutes. Stir in honey. Turn heat off and allow sauce to cool slightly before using.
