Meanwhile, in 12-inch skillet, melt remaining 1/4 cup butter over medium-high heat. Cook mushrooms, leek and garlic in butter 3 minutes, stirring occasionally, until tender. Reduce heat to medium. Stir in flour. Gradually add milk; heat to boiling. Cook 5 minutes, stirring constantly, until thickened. Remove from heat; stir in 1/2 cup of the cheese.