In a very large skillet, melt butter over medium-high heat. Add mushrooms. Cook, stirring occasionally, until softened and starting to brown, 4 to 5 minutes. Add shallot and garlic; cook, stirring occasionally, until shallot is tender, 2 to 3 minutes more.Advertisement
Sprinkle flour over vegetables in pan and stir to combine. Cook 1 minute. Carefully add stock, stirring as you pour. Add milk, sage, salt and pepper; stir to combine. Bring to a simmer. Reduce heat to medium-low and continue simmering, stirring occasionally, until gravy reaches desired thickness, 1 to 5 minutes. Taste for seasoning. Remove from heat and serve immediately.