1Peel potatoes and cut into bite-size cubes. Prepare leeks: remove dark green leaves, cut in half lengthwise without cutting all the way through; thoroughly wash the white and light green part under cold water to remove any grit. Slice the leek thinly.
2Place potatoes and leek in a large pot of boiling salted water; add onion and garlic. Cook, covered, about 15 minutes.
3Drain into a colander and set aside.
4Cook bacon until crispy; crumble and set aside.
5In the same large pot, melt butter; add flour and salt and pepper. Blend with a wire whisk, and add milk gradually, stirring constantly with whisk.
6When mixture begins to thicken, add the potatoes and leeks back into the pot, stirring until blended.
7Add the crumbled bacon to the soup; mix well.
8Add the cheese to the soup and mix well. Stir frequently until cheese is melted and soup is heated through.
9Spoon into bowls and top with croutons if desired.