Heat oil in a deep frypan over medium-high heat. In 2 batches, add chicken and brown all over for 3-4 minutes. Remove from the pan and set aside.
Reduce heat to medium, add onion, celery stalk, garlic and thyme, then season. Cook for 5 minutes, stirring, until vegetables soften. Return chicken to pan with stock, bring to the boil, then cover and simmer over low heat for 10 minutes or until chicken is cooked.
Stir in spinach, celery leaves and lentils, then cook for 3-4 minutes until spinach is wilted and lentils are warmed through. Serve with crusty bread.