All measurements here are approximately off the cuff
Measure one cup of uncooked barley. Rinse it, add one tsp salt and cover with water so there's about 2cm of water over the top of the barley. Bring to a boil and let this simmer for about 35min. There should be very little water left, and the barley should be cooked.
Take out about half the barley and keep for next time (you can freeze this). Add enough broth so it barely covers the remaining barley. Use a hand blender in this, or put it in a blender and give a few whirrs, but it doesn't have to be smooth.
In another (bigger) pot on medium heat put in a couple of TBSP oil,
one small diced onion,
one small diced potato,
one diced celery stalk,
two diced carrots,
about 10 white or crimini mushrooms sliced,
1/8 tsp turmeric
When this is all fried add the blended broth and barley mixture, and let it simmer till the vegetables are soft. If the barley mixture is really smooth, you can add a few tablespoons of the whole barely from your reserve. Also at this point thin it to your satisfaction with extra water or broth. Taste for salt and pepper, and add any herbs you like (thyme, oregano, basil, a tiny bit of nutmeg, parsley...)
Now, in a small pot melt 3 TBSP butter, then add about 1/4 cup flour and whisk furiously. Add about a cup of milk and keep whisking till you have a smooth mixture. Pour this into the soup. If you have any cream or half-and-half on hand put in about 1/4 cup of that too. Let it all simmer very gently for about 15 min more.