Melt the butter in a large skillet over medium heat. Add the leeks and sauté until they are tender, about 3-5 minutes. Add the mushrooms and continue to sauté until they just start to soften. Do not overcook. Remove ¼ cup of the mushroom mixture to use for garnish.
Add the flour, mix with the leeks and mushroom mixture, and cook for 2-3 minutes. Add the chicken broth and white wine and bring to a soft boil to thicken. Reduce the heat and add the half-and-half, cooking just until warm. Garnish each serving with a spoonful of the mushroom and leek mixture.