Step 3
Melt butter in a Dutch oven over medium heat. Add leeks; cook, stirring often, until softened, about 6 minutes. Sprinkle flour over the leeks; stir well. Cook, stirring constantly, for 2 minutes. Whisk in milk in a slow stream and cook, whisking constantly, until thick and bubbling, 8 to 10 minutes. Whisk in thyme, salt, nutmeg and pepper. Remove from the heat.