Spinach and sorrel soup

At this time of the year, both spinach and sorrel are flourishing in the garden, and we have them in all sorts of ways: the spinach mostly cooked but occasionally small leaves in salad, the sorrel mostly raw in salads, but occasionally also made into sauce for fish. But last night I combined both in a Russian-style spinach and sorrel soup, light, nutritious and tasty and just right for a late spring dinner entree.
Here's how to make it(makes enough for 4 people). Finely slice a couple of potatoes and half an onion. Saute in some butter, with some crushed garlic. Add the washed spinach and sorrel(much more spinach than sorrel--only a small handful of sorrel will do.) Stir around. Add chopped garlic chives and parsley. Add a splash of white wine, then good stock--chicken or good vegetable stock. Add pepper, salt if needed(check, though, as stock is often salty). Bring to a boil then simmer for about 25 minutes. Take off the heat and either mash the vegetables with a potato masher, as I did, or whizz in a blender. In a bowl, mix one egg yolk and about three tablespoons milk, stir, and then add to the hot soup. Serve with chives or any other herb you want.
Simple and very tasty! I recommend making it a few hours before you eat it as then the flavours have time to develop.