Melt the lard in a saucepan and cook onion in it, covered, for about 10 minutes.
Add the drained sorrel and cook, covered, over very low heat till sorrel is almost soft.
Mix sour cream and flour and stir into cooked sorrel. Simmer for 2 or 3 minutes.
Force the vegetable through a sieve or purée in a blender. Put it back over very low heat. Add sugar and simmer for another 2 or 3 minutes.
If the purée is too thick, thin it with a fewtablespoons of chicken broth or water. Then just before serving whip the purée to make it smooth and creamy. Serve with a simple roast or with boiled meat or poultry.