Heat olive oil in a large heavy-bottomed pot over medium heat. Add onion and sauté until translucent and soft, 5–7 minutes. Stir in garlic and cook 1 minute until fragrant. Add celery and sauté 3–5 minutes to soften and build flavor.
Add carrots, then pour in broth. Add bay leaf and thyme. Bring to a gentle boil, reduce to low, cover, and simmer 25–30 minutes until carrots are tender. Skim any foam for a clear broth.
Stir in cooked grains and simmer 5–10 minutes to heat through and absorb flavor. Season with salt and pepper to taste, layering seasoning as needed. Remove bay leaf and serve; garnish with chopped parsley if desired.