Peel and finely chop onion. In a small, heavy-based saucepan, melt butter over low heat. Sauté onion until golden.
Add wine, increase heat and bring to a boil. Continue boiling until liquid is reduced to about half its original volume.
Stir flour into sour cream. Slowly add sour cream to hot liquid, stirring constantly. Bring back to a boil briefly. Remove from heat and strain through a fine sieve; discard onion. Add lemon juice and season to taste with salt and pepper.