Heat oil in a large skillet over medium heat until hot. Add onion, celery and garlic; cook and stir 3 minutes or until tender. Remove vegetables with slotted spoon to a small bowl.
Arrange chicken in single layer in skillet. Cook over medium-high heat 5 minutes per side or until lightly browned.
Add onion, celery, garlic, carrots, parsnips, potatoes, thyme, salt and pepper to the skillet. Pour stout over chicken and vegetables. Bring to a boil over high heat. Reduce heat to low. Cover and simmer 35 minutes.
Add mushrooms and corn to skillet. Cover and cook 10 minutes. Then uncover skillet; increase heat to medium. Cook 10 minutes longer or until sauce is slightly reduced and chicken is no longer pink in center.