Simmer: Heat over medium-high heat until it starts to thicken, then reduce to medium. Once thickened to desired consistency, remove from heat and stir in remaining ½ cup milk (this will help maintain the desired consistency because it thickens when removed from the heat). If the gravy becomes too thick, whisk in additional milk, if it’s too thin, then return to heat. Taste and season with salt and pepper to taste.