Heat the butter in a pan, add the flour and roast a little (but should stay light). Pour the cold vegetable stock slowly while stirring, bring to the boil, let simmer for 10 minutes on a low heat.
Add the cress, parsley and garlic. Let the whole thing simmer for 15 to 20 minutes over a low heat, stirring occasionally. Puree the soup well with the hand blender and pass through a sieve into another pan.
Heat the soup, but do not let it boil. Mix the egg yolks and cream together and add to the soup to mix. Season to taste with salt, pepper, sugar and a little lemon juice.