01 - Preheat the oven to 375°F.
02 - In a large oven-safe Dutch oven or deep skillet, heat olive oil over medium heat. Add onion and sauté for 3–4 minutes until translucent. Stir in garlic and cook for 1 minute until fragrant.
03 - Add mushrooms and thyme; cook for 6–8 minutes, stirring occasionally, until mushrooms are browned and have released their moisture.
04 - Stir in barley and toast for 2 minutes.
05 - Pour in the white wine, stirring to deglaze the pan. Cook for 2 minutes until mostly absorbed.
06 - Add the warm vegetable broth, salt, and pepper. Stir well, bring to a simmer, then cover with a tight-fitting lid.
07 - Transfer to the oven and bake for 40 minutes, stirring halfway through, until barley is tender and most liquid is absorbed.
08 - Remove from oven. Stir in spinach, Parmesan, and butter. Cover and let rest 5 minutes for flavors to meld.
09 - Taste and adjust seasoning. Serve warm, garnished with parsley and extra Parmesan.