Melt the butter in a large Dutch oven over medium heat. Add the oatmeal and lightly toast it for 1-2 minutes.
Stir in the leeks, broth, milk, salt, and pepper. Bring the mixture to a boil.
Reduce the heat to low-medium and simmer gently. Cover and cook for about 40 minutes, or until the oatmeal is tender.
Serve hot, optionally garnished with aged white cheddar cheese. (Trust me, the cheese takes the flavors to the next level!)