Heat olive oil in a large pot over medium-high heat. Season chicken with salt and pepper, then sear until browned on all sides. Remove and set aside.
Add onion, celery, and carrots to the pot. Sauté for 5–7 minutes until vegetables are tender.
Stir in garlic and cook for another 1–2 minutes until fragrant.
Pour in chicken broth and add potatoes, thyme, and rosemary. Return chicken to the pot.
Bring to a boil, then reduce heat to a simmer. Cover and cook for 30 minutes, stirring occasionally.
In a small bowl, whisk flour with 1/2 cup of the stew liquid to form a slurry. Stir it into the pot to thicken the stew.
Add frozen peas and simmer uncovered for another 10–15 minutes, until the stew thickens and everything is cooked through.
Garnish with parsley before serving, if desired.