Melt the butter in a large Dutch oven over medium-low heat.
Add the leeks, increase the heat to medium, cover, and cook, stirring occasionally, until the leeks are tender but not mushy, 15-20 minutes.
Do NOT brown them.
Add the flour and cook, stirring constantly, until thoroughly combined, about 2 minutes.
Increase the heat to high; whisking constantly, gradually add the broth.
Add the potatoes and bay leaf, cover.
Bring to a boil.
When soup reaches a boil; reduce the heat to medium-low and simmer, COVERED, until the potatoes are ALMOST tender, 5-7 minutes.
You can try to push a fork into the potatoes to test their doneness.
You should feel some resistance, but the fork should slide in easily.
Remove the pot from the heat and let stand, COVERED until the potatoes are tender, 10-15 minutes.
Discard the bay leaf, season with salt and pepper to taste.
I cut fresh chives from the garden and shredded 2 Tablespoons of sharp cheddar cheese to garnish.