Heat oil in a 6cm-deep, 22cm (base) and 25cm (top) heavy-based, ovenproof frying pan. Add chicken. Cook for 5 minutes or until just browned. Add leek, celery and carrot. Cook, stirring occasionally, for 5 minutes or until leek is tender. Add garlic, rosemary, thyme and sage. Cook for 1 minute or until fragrant. Add our. Stir to coat. Gradually stir in stock and milk. Bring to the boil, stirring occasionally. Remove from heat. Stir in corn. Season with salt and pepper.