Warm olive oil in a large pot over medium heat.
Soften onion, carrots, and celery with a pinch of salt for 5 minutes.
Stir in garlic and thyme; cook until fragrant, about 30 seconds.
Pour in broth and add chicken; bring to a gentle boil.
Rinse rice, then stir it in. Reduce heat, cover, and simmer 15–18 minutes.
Lift out chicken, chop or shred, and return it to the pot.
Stir in peas, lemon juice, parsley, and black pepper; simmer 2 minutes.
Taste and adjust salt. Let soup rest 5 minutes before serving.