01 - Preheat the oven to 425°F. Line a large baking sheet with parchment paper if desired.
02 - Toss the quartered mushrooms with olive oil, sea salt, black pepper, and thyme leaves directly on the baking sheet. Spread in an even single layer and roast for 20–25 minutes, stirring halfway through, until the mushrooms are golden brown and tender.
03 - While the mushrooms roast, heat 1 tbsp olive oil in a medium saucepan over medium heat. Add the finely chopped onion and sauté for 3–4 minutes until softened and translucent. Add the minced garlic and cook for 1 minute until fragrant.
04 - Stir in the rinsed brown rice, dried Italian herbs, salt, and pepper. Toast the rice for 1 minute to coat in the oil and release the herb aroma. Pour in the vegetable broth, bring to a boil, then cover with a lid, reduce heat to low, and simmer for 35–40 minutes until the rice is tender and the liquid is fully absorbed. Remove from heat and let stand, covered, for 5 minutes.
05 - Fluff the rice with a fork. Fold in the roasted mushrooms, chopped parsley, chives, lemon zest, and lemon juice. Taste and adjust seasoning with additional salt and pepper as needed.
06 - Transfer to a serving dish and serve warm. Top with freshly grated Parmesan cheese if desired.