In a 2-quart saucepan over medium-high heat, cook onions in butter until soft.
Add potatoes, chicken broth, parsley, thyme, celery seed, salt, and pepper to saucepan; simmer for 15 minutes over medium-low heat, stirring occasionally.
Puree half of soup in a blender; return puree to the saucepan.
Melt butter in a sauce pan. Add flour and stir until smooth and bubbly. Remove from heat and add the milk. Return to heat stirring constantly until thickned and boiling. Boil for one minute.
Add to soup and simmer about 5 minutes.
Notes:
No celery seed? Use celery salt and omit or reduce added salt.