In a deep sauce pot, brown the bacon until crispy. With a slotted spoon, remove bacon to paper towels to drain. Pour any excess fat from the pan, leaving about 1 tablespoon. Saute the leeks, celery, thyme, and bay leaf until the vegetables start to soften. Gradually add the chicken stock, and salt and pepper, to taste. Simmer for about 30 minutes, then remove bay leaf and blend vegetables right in the pot with an immersion blender. Create a beurre manie (reverse roux)
by blending the butter and flour together until they look like coarse crumbs. (Flour molecules would normally clump together in a hot liquid. This combination of butter and flour works to thicken already cooked sauces without making them lumpy.) Add this to the soup and blend again until smooth. Adjust seasoning with salt and pepper, and serve garnished with chopped chives.