Soak dried shiitake mushrooms in a bowl of hot water for 20 minutes.
In a separate bowl, soak rinsed barley in hot water for 20 minutes.
*After 20 minutes* Once they're soaked, drain the mushrooms (do not discard the soaking water) and slice them into pieces. Also drain the barley. Set aside.
Set a cooking pan over medium heat. Add oil. Once it's hot, add garlic + onion and stir-sauté for 2-3 minutes, or until translucent.
Add the button mushrooms + shiitake mushrooms and stir-sauté for 10 minutes, or until much of the moisture is gone.
Now remove and transfer around 1 cup of this mixture to a blender and blend (with the soaking mushroom water from earlier) until creamy. Transfer it back to the cooking pan.
Add rice vinegar + salt + pepper + barley + water to the pan. Stir-mix and cover the pan. Cook over medium heat for 15-18 minutes, or until the barley is soft. Make sure you give it a quick stir every 4-5 minutes.
Once cooked, add freshly chopped parsley / cilantro and mix. Creamy, umami mushroom barley risotto is ready. Transfer to serving dishes and enjoy while it's still hot.
If you want your risotto more creamy, simply blend more mushrooms. However, you'll need to stir more frequently while it's cooking (and under a lower heat - there is going to be a lot of splatter with a thicker/creamier risotto).