Sauté the onion, lettuce and sorrel, stirring gently for 2-3 minutes, in a non-stick pan.
Add the stock and simmer, covered, for 5-6 minutes. Blend in a liquidizer.
Return to the pan and stir in the cottage cheese mixed with the egg yolk; heat gently, stirring, until thickened. Do not let the sauce boil. Season to taste with salt and pepper.