Slice and clean leeks in water, allowing silt to fall to the bottom of a bowl of water and lifting the floating leek slices from the top of the water. Cube potatoes in 1/2 inch dice.
Melt butter in large pot. Saute leeks and potatoes for 7-10 minutes until beginning to soften. Stir in flour to coat vegetables. Slowly add broth, stirring continuously. Add herbs and spices. Simmer 30 minutes.
Whisk remaining flour into milk. Stir into the simmering soup. Cook five minutes. Serve.