In a 12-inch skillet over medium, heat the butter and garlic until the butter has melted and the mixture has begun to sizzle. Add the mushrooms and shallots and cook, stirring, until the mushrooms have released their liquid and the shallots have softened, about 5 minutes. Add the sausage and cook, stirring and breaking the meat into small pieces, until no longer pink, 8 to 10 minutes. Discard any accumulated fat.