Heat 1 tablespoon of oil in a large pot over medium heat. Add onions and a pinch of salt. Reduce heat to medium-low and cook until onions are golden brown and melty, about 20 minutes. (If the bottom of the pot is getting too dark while the onions cook, add a splash of water, scrape up the brown bits and keep cooking.) Add 1 cup of water to the pot, scraping up the brown bits that have formed on the bottom with a wooden spoon. Transfer the onions and liquid to a bowl. Set aside.

Heat remaining tablespoon of oil in the now empty pot over medium. Add mushrooms and a pinch of salt and cook until mushrooms are browned and tender, about 10 minutes. Add garlic and thyme and cook for one more minute. Add sherry and simmer until bubbly and reduced slightly, about 1 minute. Add broth, barley, and caramelized onions/liquid mixture to the pot. Bring to a boil over high heat and then reduce to a simmer. Continue to cook until barley is tender, about 35 minutes, adding additional water as needed if soup becomes too thick. Add soy sauce, sherry vinegar and parsley and season to taste with salt and pepper. Serve.