In a large pot or dutch oven, melt the butter on medium heat, then add the leeks and garlic. (If adding carrots and celery, add them now as well)
Saute the vegetables until the leeks have softened and become fragrant.
Add the potatoes, ham, chicken broth, dill, salt and pepper. Increase heat, bring to a boil, then reduce to a simmer and cover for about 20 minutes, until potatoes have begun to soften.
Add 1 cup of milk, then whisk the flour with the remaining ½ cup of milk and add to the pot. Stir well and continue to heat until the soup has thickened
Serve with a sprinkle of fresh herbs or green onions.