Preheat oven to 350°F. In the same skillet you cooked the sausage in, melt butter on medium-low. Add garlic and cook, stirring frequently, for 1 minute, until fragrant. Gradually whisk in flour and cook, whisking constantly, for 2 minutes. Slowly whisk in 1 cup milk. Increase heat to medium-high and simmer, whisking frequently, until slightly thickened, 2 to 3 minutes.