1/2 cup coarsely minced onion, lightly caramelized in oil (butter if you want). Meanwhile:
1 medium potato, diced (1/2 to 3/4" pieces) simmering in a scant qt. of chicken stock.
Add a rind of Grana Padano. Parmesan would be good too.
1/2 cup of celery pieces about 1/2 to 3/4" long, added to the stock.
Once the onion caramelizes, put it in too, then use the same skillet to make a roux with 1 oz. of butter and enough flour to make.
Cook the roux only long enough to get past the "raw flour" taste (longer if you want it brown), then cool it to room temperature or below.
(There'll be no lumps if hot roux is added to cold liquid or vice versa.)
Add 2 or three cloves of garlic, minced, to the soup and simmer a bit more.
When ready to serve, stir in the roux a bit at a time. Add ground pepper to taste. White pepper avoids black specks.