Chop onion and fry in the half a butter. Add flour and fry gently. Pour the broth, cream and dry white wine into the saucepan, cook for 3 minutes, stirring occasionally. Season with salt, pepper and a pinch of sugar . Wash the wild garlic, dry and chop, add in the soup, heat gently and mash in a blender. Dice toast bread and fry in the butter. Whisk egg yolks with cream, pour over the soup, season to taste, sprinkle with lemon zest and serving.