Place flour into a large snap-lock bag. Season. Add chicken and toss to lightly coat in flour.
Heat oil in large ovenproof pan over a medium-high heat. Brown chicken in 2 batches. Transfer to a plate. Set aside.
Add onion, garlic, carrots and celery to pan. Cook, stirring often, over medium heat for 4-5 minutes until onion softens. Stir in mushrooms. Cook, stirring occasionally, for 2-3 minutes until slightly softened.
Stir in stock. Return chicken to pan. Add thyme. Bring to the boil. Reduce heat, cover and simmer, stirring occasionally, for 15 minutes. Uncover and cook for 15 minutes or until chicken is just cooked through. Mix cornflour with 2 tsp water in a cup until smooth. Stir cornflour mixture into chicken mixture. Cook for 1 minute or until thickened. Stir in peas and cook for 2 minutes. Serve with mashed potatoes, rice or couscous.