Prep your vegetables. Slice the mushrooms and celery thin for a softer texture. Finely chop the onion, mince the garlic, and dice the carrot if using. Have the thyme and bay leaf ready.

Warm the pot. Heat the olive oil in a large soup pot over medium heat. When shimmering, add the onion and a pinch of salt. Cook 3–4 minutes until translucent, stirring often.

Sweat the celery and carrot. Add celery (and carrot if using). Cook 5–6 minutes, stirring occasionally, until the vegetables soften but don’t brown. This gentle heat keeps flavors calm and sweet.

Add mushrooms in batches. Stir in half the mushrooms and let them release their liquid, about 3–4 minutes. Add the remaining mushrooms and a pinch more salt. Cook until they shrink and smell earthy, 4–5 minutes.

Layer aromatics. Add the garlic, thyme, bay leaf, and red pepper flakes if using. Cook 30–60 seconds until fragrant. Don’t let the garlic brown.

Pour in the broth. Add the vegetable broth and scrape the bottom of the pot to release any browned bits. Bring to a gentle simmer, then reduce the heat to low.

Simmer softly. Let the soup cook for 15–20 minutes. Keep it at a low simmer so the flavors meld without becoming muddy or harsh.

Finish bright. Remove the thyme stems and bay leaf. Stir in parsley, lemon zest, and a squeeze of lemon juice. Taste and adjust with salt, pepper, and more lemon as needed.

Optional grains. If using cooked brown rice or barley, stir in now and warm through for 2–3 minutes.

Serve. Ladle into bowls and top with extra parsley or a drizzle of olive oil. Enjoy warm.